Undertale: The Human Equation
by Mycatisthebestsowhatever
Summary: The misadventures of Sans and Frisk. After Sans makes the decision to ultimately become human, he ends up asleep for five years. When he wakes, Frisk is an adult. Both love each other, but through trial and tribulation they must learn both how to handle a world with many diverse creatures, and learn what it means to be human
1. chapter 1: The Decision

Undertale: the human equation

\- hey there. Welcome to my little undertale fanfic. I have no idea how long this story will go, but updates should happen bi-weekly. Generally Mondays or Tuesdays will suffice, but we'll see. This will not involve any of the other undertale fanfics, such as underfell or others. I will put a trigger alert here, however, because I am relentless with my writing and will probably dabble into some darker stuff later on. This first chapter should be light reading for all of you, and I hope you enjoy. I will switch back and forth between San's perspective and Frisk's. It just seems more fun that way. I'll probably make some art, too. It'll all be hand done, as I'm not the best with a tablet.

Chapter 1: the decision

Golden sunlight fell upon green, grassy lands. Silhouetted against the sun were mountains in the distance. The sun rose to greet seven creatures standing alongside a giant hole in the ground. A great battle had ended, and the creatures were rejoicing in their newfound freedom. As happy as the scene was, one such creature stood in uncertainty.

He was a short skeleton with a lot of magic at his fingertips and even more worry in his chest. The king and queen of the underground, two goat figures stood beside a human with smiles on their faces. Alphys the pudgy lizard woman stood near her new lover Undyne, who held her scaly fist against the sun. Papyrus laughed and patted the human on the back, stating loudly that Frisk couldn't have done anything without _his_ help.

Sans, of course, knew better. He had watched the different timelines form when he saw the time anomalies in his lab back at the house. He had even shown his brother, in an act of desperation, when he feared that the human might try to kill them.

But she hadn't. Frisk hadn't killed a single creature, even the enraged dummy. She had spared all of them, thus saving them without fighting at all. Frisk opened her mouth wide and laughed at Papyrus, before turning her head to look at Sans. She was blind, so she couldn't _actually_ see, but somehow Sans felt that she could sense him, and everyone.

Sunlight illuminated her small face as she grinned.

"I, the great Papyrus, will be the best mascot," Papyrus said triumphantly. "I'll go make a good first impression!" He quickly ran away into the nearby forest.

As usual, the power of the human sent chills through San's bones. He kept his gaze locked on her, before winking and grinning. "Someone's gotta keep him from getting into trouble." He turned on his heel and headed towards the ruins. Sans did this for two reasons: the others, aside from Alphys, would find it funny. He also wanted the human to know that he, too, had certain powers beyond what the others could do.

In truth, Sans needed to exit the scene. He wasn't upset, if anything he was incredibly relieved, but he knew as well as Papyrus did that if the human _really_ wanted to, they could go back and erase everything. San's magic burned blue and he teleported back to his house, in Snowden.

A year had passed. During that time, Frisk and Toriel made frequent visits to the surface. Toriel never admitted it, but she went to protect Frisk from other humans. They had began speaking with the chief of the nearest village, who were responsible for the wars anway. Papyrus had gone with them once or twice, to make jokes and supposedly to cook. Sans teleported once just to check, only to see Frisk and the humans smiling and happy with Toriel and Paps.

At the time, no one had noticed Sans, and he was about to leave, unnoticed. But, right as his magic began to burn at his fingertips, Frisk turned her little head and caught his eye.

She smiled and waved, and even beckoned Sans forward. Sans felt his bones tense at the sight of the girl. He couldn't even shake his head, he merely teleported home and collapsed on the floor of his dirty room, clutching his clothes.

 _I don't have a heart,_ he thought, _but would it be beating fast?_

It had been a year, full of surface visits, monsters moving out from the underground and humans moving in, and random, new encounters. The human came to see him, sometimes, and Sans did his best to make things fun for the child. He had noticed, however, a little yellow flower sometimes and grew worried. Ultimately, needing space, he isolated himself to his room.

But, of course, he couldn't really be alone.

The dirty floor of his room suddenly cracked and groaned and a flower burst through. It looked left and then right.

"Yuck, when was the last time you cleaned in here?" Flowey asked.

Sans shrugged. "I guess I just don't have the _heart_ to clean it."

Flowey snickered and moved forward. "Listen, here, bone head, I've been watching you."

"For someone who watches the timelines as much as I do, how can I say I'm not surprised?" Sans replied dryly.

Flowey's normal grin fell to a hard frown. "I need to ask you something."

Sans sighed and closed his eyes. "Sure, but can I take a nap first?"

" _No,_ dammit!" Flowey moved his roots and used one to snap at Sans's legs.

"Hey now, no need for that," Sans said, rubbing his leg. He winked. "And here I was, thinking you had _grown_ a little."

"Ha, ha, wordplay," Flowey said, wiggling his stem. "Hey. Do you have a crush on Frisk?"

Sans froze and felt his bones tremble. Flowey was the only person who could warp space and time other then Sans himself. If Sans answered in the wrong way, Flowey might try to destroy the entire timeline.

"Your stunned look is telling me yes," Flowey speculated thoughtfully.

Sans's heart fell and he shook his head. "You've got it all wrong."

"I am? Wow, and her I was about to warn you that she's marching over here to confess," Flowey said with a wicked grin.

Sans froze again and his chest tightened. _She's gonna confess?_ That sounded similar to something Alphys would watch in her lab.

"You _do_ like her!" Flowey cried.

"Hold on," Sans started.

"She's just a kid!" Flowey spat. "You're disgusting.'

Sans was quick with his words. "Well," he said with a laugh, "I'm not really that old, I mean technically I didn't appear until about two years ago."

"That's a flimsy excuse, besides," Flowey was grinning awfully again. "You're a skeleton. You'd never be able to be with her."

Flowey was saying something that San's hadn't quite come to terms with. His heart fell and he looked at his feet. From outside of his room, Sans could hear Papyrus at the door.

"Human! You've come for a visit! And I, your best friend, the great Papyrus, will entertain you in the best of ways!" Papyrus stomped through the living room.

 _Ah shit, she really is here,_ Sans thought in a panic.

"Is Sans here?" he heard Frisk's small voice.

"If he isn't being a _lazy bones!_ " Papyrus yelled.

Sans put his hand on his head and tried to think.

"Good luck, lazy bones," Flowey said with a malicious laugh and left, leaving a little hole in the center of Sans's floor.

"Paps, could you make some spaghetti? I'll go get Sans," Frisk said.

"Yes! And it will be the most fantastic spaghetti you will have ever had! Even better then the frozen spaghetti I left on that table!" Papyrus cried, cackling and walking into the kitchen.

Sans heard Frisk walk up to his door. She knocked softly and waited.

Sans momentarily contemplated just ignoring her, before he noticed a brown root rise out of the floor and turn the handle.

 _Damn flower,_ Sans thought.

Frisk stood in her too-large shirt with a cheesy smile on her face. Most other monsters either thought she was a boy or didn't bother recognizing her gender, but Sans knew.

"Hey bone head," she said with a toothy grin. She stepped forward confidently.

"Hey, kid," Sans said, doing his best to appear normal.

Frisk sat beside him on his bed and stared at his swirling vortex of paper and garbage in the corner. "You didn't come to my birthday party."

 _Shit._ "Ah, kid," Sans said. "I'm sorry."

"I can't forgive you," Frisk said stubbornly and crossed her arms.

 _Oh, no._ Sans felt miserable and looked around his room, and thought of all the places he could go in the underground to get her a gift.

"At least, not unless you can answer me one question," Frisk went on.

"Alright," Sans replied cooly.

"Who has two thumbs, and likes you a whole bunch," Frisk asked. Her cheeks ran red.

Sans was taken off guard and stared at the hole in his floor where Flowey had been. Before his own assumptions could get the best of himself, he stuck out both thumbs and pointed at himself. "This guy," he said.

Frisk giggled and shook her head. "It's me, you dope," she said and playfully punched Sans on the shoulder.

Sans sighed and continued to stare at the floor. "Kid," he started.

Frisk hopped off of the bed and walked towards the door. "That's all I wanted to say," she told him. "No need to get mushy, I know I'm quite the catch." Frisk grinned at Sans before she left.

Sans lifted his finger and his door shut. "Kid, we need to talk."

Frisk blushed and nodded, but didn't move from the doorway.

"I'm a whole heck of a lot older than you," Sans said.

"I don't care," Frisk started but Sans held up his hand.

"Well, I do," Sans said sternly. "It doesn't matter if we like each other. We can't do anything."

Frisk pouted and looked away.

"Second...I'm not human, Frisk." Sans sighed and felt his body sag. "Even if... _I'm_ not so old, my memories and my knowledge is. All the things I learned working with Alphys and Gaster...all the timelines I know of. I'm not human, and it wouldn't be fair. I could use magic to simulate what humans do, but...it wouldn't be the same. Especially with you being so young."

"I'm mature for my age," Frisk argued. "Besides, I'm officially a teenager. I'm thirteen now!"

Sans stood and patted Frisk's shoulder. "Kid. If...you really want to be with me, then you're gonna have to wait a least five years." Sans used his magic to open the door. "And that's a long time."

"I'll do it."

Sans froze and stared at Frisk.

"Kid," he started. "Don't."

"Are you saying you don't like me, then?" Frisk asked. Her little fists clenched and she stood her ground.

"I'm saying that for you, five years is a long time," Sans insisted. "You'll have plenty of boyfriends and I don't want to get in the way of that."

"You won't," Frisk said determinedly. "This is my choice."

Sans stared at her, tempted to hug her or kiss her, but he steadied himself. "If you change your mind, it's okay." Sans gestered for Frisk to leave.

Frisk stepped through the doorway and paused. Both could hear Papyrus singing a song downstairs.

"Sans?" Frisk said, and turned. She opened her eyes, even though she couldn't see.

"What's up, kid?" Sans asked.

"I love you."

Sans froze again, only this time, the urge to kiss her almost overtook him. Still, something in him cringed at the wrongness, and his own morals screamed the reality that she was just a child. A small voice in his mind said: so are you, but he ignored it. Sans knew better. And as he stared at the child, pouring her feelings out to him, he knew that he had to do something.

Sans reached forward and patted Frisk's head. "Kid...I love you too. Now stop being silly and quit ignoring Paps."

"Human! The food has been prepared!" Papyrus announced from the kitchen.

Undyne had once told Frisk that she had to be assertive. This was not uncommon or even unknown to the young girl, because in order to save Aserial, she had to continue standing up for herself. So, when she realized that she loved Sans, she did the only thing she knew she _should_ do. She told him. And she received no answer from the short skeleton.

Frisk wandered the ruins the next day, talking with the frogits and laying with Nabstablook. She had gotten him a monocle which he adored and wore with his hat when he was in good spirits. After the ghost creature faded away, Frisk found herself walking alone.

Though some monsters now lived on the surface, and some humans now lived underground, the ruins remained empty for most except for Toriel. She and Asgore had been speaking for several months, and sometimes even going out to Grillby's, but she remained in her own space instead of living with him. Frisk never questioned this, because it seemed like a sensitive subject. Even so, Frisk liked to pretend that someday the king and queen would be together again.

Toriel sat in her chair next to her fireplace, reading a book. Her furry nose was always cute to Frisk, but as Frisk sat down, she couldn't relish in anything.

Did Sans like her? Or not?

He had said he loved her, but was that more of a family statement, or did his feelings burn as strongly as Frisk's?

Toriel stood and gracefully walked to Frisk's side. She put a fuzzy paw on the child's shoulders.

"You look troubled," she said gently.

Frisk stared at the kind Toriel before letting out a long sigh.

Toriel nodded, as though she expected Frisk's actions. "Would you like to talk about it?" she asked, settling in her chair once more.

Frisk tapped her fingers on the table. "I told Sans that I love him."

Toriel coughed and dropped her book. "Oh," she said, with raised eyebrows. "Really." Frisk couldn't help but detect a hint of anger.

"He didn't do anything," Frisk said dryly.

"I would still very much so like a word with him," Toriel said flatly, putting her chin on her fist.

"He said that he was too old and that he wasn't human so we couldn't do anything," Frisk said. Her nose felt hot and she turned away to hide her stinging eyes. Toriel wasn't looking, but Frisk was already uncomfortable talking about her feelings to what she considered as her mother.

"Even so," Toriel said. "He is...kind, but he can be very adult regardless of his actual age."

"His actual age?" Frisk asked.

"He and Papyrus just appeared one day," Toriel said. She offered a sly smile. "He came to my door and began telling me jokes after that."

Frisk sighed and shook her head.

"Oh my sweet child," Toriel said. "Not all romances happen the way we want them to." The wisdom in her eyes made Frisk a little sad.

"I know we could be happy together," Frisk said stubbornly. "Why is that bad?"

Toriel sighed and ran a hand over the top of her head. "Sans...is doing what is best for you."

Frustration blossomed in Frisk's chest. " _I_ know what's best for me."

Before Toriel could answer, her phone wrang. She hesitated before answering. "Hello?" she cooed as she stood. Toriel walked to Frisk and patted her shoulder.

Frisk watched curiously, hearing the sharpness of Alphys's voice. As Alphys spoke, Toriel's brows rose.

"What? But…"

Alphys went on, and Frisk could practically hear the anxiousness in Alphys's voice.

"Alright. Well, call...yes, sorry, text Asgore immediately." Toriel looked at her phone with furrowed brows.

"Tori?" Frisk asked. "What is it?"

Toriel looked stricken at first, before pocketing her phone. "Nothing, my child. I've made some butterscotch pie. I'll bring a slice up to your room if you'll wait there for me."

Frisk nodded, and trudged through the house while Toriel began to text quickly. Frisk pulled out her own phone, and texted Alphys.

 _What's going on?_ Frisk was sure she'd wait for a while before she got an answer. To her surprise, Alphys answered immediately.

 _Sans is trying to become a human._


	2. Chapter 2: Sans's Last Thought

Undertale: the human equation

chapter 2: Sans's last thought

Frisk:

Frisk stared at her phone incredulously for several seconds, before turning to Toriel. Toriel put her phone in her pocket and swiftly began walking. Frisk's breath caught in her throat and she suddenly dashed through the house, swiftly dodging Toriel.

'Frisk!' Toriel cried.

Frisk dashed down the stairs and hurried through the familiar ruins. As she made her way out into the snowy forest just before snowden, her phone rang. In mid run, she answered it.

'Yes! Um, human?'

'What is it, Paps?' Frisk demanded breathlessly.

'Well, Sans was telling me just a few minutes ago that he's going to need some butterscocth pie,' Papyrus said.

Frisk couldn't help but grin. 'Did he want that before or after he turns human?' she asked.

'Oh, so you already know!' Papyrus said joyfully. 'Did you hear that, Sans? Frisk already knows! Oh, Frisk, I'm so happy, Sans didn't want me to tell you what was going on because he thought you'd try to run after him or something, and sometimes Alphys's experiments can be a bit….explody.'

'That's exactly the problem!' Frisk cried into the phone.

'Wowie, you are very today,' Papyrus said. 'Sans seems determined, though.'

'Paps, Alphys's experiments don't always work,' Frisk said. She nearly tripped over the pet rabbit in snowden, and ran to the river towards the ferry.

'I'm sure it'll turn out fine, besides, Mettaton works just fine,' Papyrus said with a _Nyeh heh heh._ In the background, Frisk heard Mettaton shout: ' _Oh yeeeees._ '

'Papyrus, listen to me,' Frisk said frantically.

'Everything will be fine. And if Sans becomes human, maybe I can, too!'

'Paps,' Frisk groaned.

'Just go eat some pie. Sans is saying that you shouldn't worry, but you shouldn't come over,' Papyrus said. 'Alphys is saying something to me now. Oh! I need to go.'

'Paps, wait!' Frisk cried, but the call ended. Frisk looked at the ferryman, who offered a shrug. 'Can you take me to hotland?' Frisk asked.

'Yes,' he said in a low whisper.

Sans was laying on a flat table staring at the florescent lights in the Lab. Alphys scuttled around while Papyrus babbled on the phone. At the sound of Frisk's name, Sans cringed but did his best to remain calm. Then suddenly:

'Did you hear that, Sans? Frisk already knows!'

 _Shit._ Sans slowly sat up, disturbing the wires and suction cups Alphys had squished against his bones.

'Oh,' Alphys said. 'S-sorry, that might be my fault.'

'How fast can we do this?' Sans asked.

'It'll take some time,' Alphys said, looking at a clip board filled with notes and information. 'Maybe a couple of days. I'll have to put you in a tube to see how the process works, and how your magic fuses to the…' Alphys glanced around before letting out a nervous laugh. 'We don't need to get anymore detailed.'

'Paps, tell her not to come,' Sans said. He was in only his shorts, and he didn't want the human to see him almost naked. At least, not before he had a body. 'I'm gonna be her age, right?' Sans asked.

Alphys laughed nervously again. 'Technically much younger, I mean it's a _new body,_ so...with your thoughts and soul, I'd think you'd just be you in a new form.'

'It's fabulous darling,' Mettaton said. 'New bodies are grand. Just look at mine.'

Sams stared at the machine, before saying: 'I'm not gonna be that, am I?'

'Whatever do you mean?' Mettaton cried.

'Well,' Alphys said. 'Mettaton made its body. And part of that process involved a deep magic within the soul of Mettaton.' Alphys offered a small smile. 'You two can use that type of magic, but it's really hard for others with bodies to use that magic. It's normally not even something anyone uses in their lifetime. But Mettaton is…'

'Absolutely special,' Mettaton said with a grin. 'Oh, sweet honey Alphys, anyone can use soul magic if they try hard enough. Just look at Asgore!'

Alphys scoweled and one of her computers started beeping. 'Oh,' she said and hurried towards it.

'Sans,' Papyrus said. 'I know you're intent on doing this whole….human thing, but maybe it's best to just be yourself? I like you the way you are! In fact, I'm sure everyone does!'

Sans chuckled. 'Is she on her way?'

'I told her to go get some pie, so I think we're in the clear,' Papyrus said with a a frown.

'She's probably on her way,' Sans said. 'Alphys we gotta get this started.'

'But, Sans, there's no harm in being yourself,' Papyrus insisted, clenching his gloved fists.

'Nah, Paps. It's important to be who you are, but sometimes things gotta change.' Sans offered his brother a grin. 'Besides I'm not going anywhere.'

Papyrus smiled and nodded, but couldn't hide his uncertainty.

'Alphys,' Sans said, 'how is this gonna work?'

'I'll have to put you into a hypnosis, and get you to focus your magic on making a body,' she said. 'Then we'll put you in a large testing tub and see what happens when we add body matter.'

'Cool,' Sans said, but he wasn't sure he completely understood.

A loud knock resounded on the entrance to the lab.

'I wonder who that could be!' Papyrus said cheerfully.

Sans began to sweat. 'Alphys let's do this.'

Alphys nervously glanced from one person to another. 'Sans, are you sure?'

'Yeah,' he replied firmly.

'A-alright,' Alphys replied.

'I'll go get the door!' Papyrus said with a grin.

'I'm with you, darling, we need an audience for Sans's transformation into a beautiful young man,' Mettaton said with a smile.

'Paps,' Sans started.

'Don't worry about them,' Alphys said. She pulled out a meddalion and began to swing it. 'Pay attention to this.'

 _Frisk,_ Sans thought, but did his best to do as Alphys said.

'Y-you're getting very sleepy,' Alphys said. She was trying to sound calm, but Sans could hear the anxiousness in her voice.

Despite that, he allowed himself to relax.

'As you get sleepier, tell me what you want to do,' Alphys said.

'I want to become human,' Sans said slowly. His eyes were becoming heavy and he wanted to fall asleep.

'Why do you want to become human?' Alphys asked. The rest of the world was becoming blurry and Sans could hear nothing but Alphys's voice.

Sans almost didn't answer. He didn't want to admit anything to anyone but himself, but his sleepiness made it hard to think. 'I wanna be with Frisk,' he said.

'W-what?' Alphys suddenly said. She blushed, and glanced around. 'Really?'

'Yeah,' Sans said slowly. 'But she's too young. I can't do that. That's wrong.'

'Why do you have to be human to be with her?' Alphys asked.

''Cause,' Sans didn't want to answer again. He was struggling to stay awake. ''Cause I can't do anything for her like this.'

Alphys shook her head. 'Okay, Sans. Sorry for derailing. Focus on what you want, which is a human body.'

'Frisk,' Sans said sleepily. 'I just want frisk...gotta wait...five years….she's just a kid…' Sans fell asleep with his magic burning in his eye socket.

Papyrus opened the door of the lab, only to see Frisk. 'Oh, Frisk!'

'If it isn't the little star that upstaged me,' Mettaton said with a sly grin. '

'Where is Sans?' Frisk demanded. 'Let me through!'

'Oh! Wowie, well you've put me in a pickle,' Papyrus said, scratching his head. 'You see, Sans told me to keep you away from the lab and I'm sure that he'd like to see you but he's in the middle of a big transformation. Alphys told me it's sort of like when a catarpillar becomes a butterfly! Did you know they did that? I'm going to watch it happen as soon as I get back to the surface.'

'Paps, get out of the way,' Frisk said desprately and tried to step around him.

Papyrus simply stepped back into Frisk's way with a smile. 'In a moment, Frisk, because I've learned so much about butterflies and I just want to share it all with you!'

'Papyrus, darling, you haven't told _me_ about butterflies,' Mettaton said and put a dramatic hand on his head. 'I'm hurt.'

'Well, if you'd like, we can go to the meadow just beyond where the barrier was and watch them together,' Papyrus said happily.

Frisk groaned. 'Will you two just move?' she demanded.

'But you can come with us! And then we can see Sans later in his new body!' Papyrus said.

'We can't let him do this!' Frisk said and pushed past her two friends.

'Frisk,darling, how rude!' Mettaton cried.

Frisk hurried into the lab and spotted Alphys staring at a large tube filled with bubbling water. She tapped a pen to her chin and held a clip board in another hand. Frisk stopped breathlessly in front of her yellow lizard friend.

She turned and jumped, nearly dropping her clip board. 'Oh, Frisk!' she said with an awkward smile.

'Where's Sans?' Frisk demanded.

'Sans? Well…' Alphys glanced at the giant tube filled with water.

'Alphys, I trust you, but the last time you did a bunch of intense experiments with bodies and souls,' Frisk started.

'I know,' Alphys started. She sighed. 'I know, Frisk. But the thing is...Sans is really powerful. It isn't just about making a physical body, it's about how much willpower and magic a monster can use.'

'Where is he?' Frisk demanded again. 'Let me see him.'

Alphys glanced at the tube of water again. She sighed and stepped forward. She took Frisk's hand in hers and stepped forward. 'I don't think we're gonna be able to wake him up,' she said.

Frisk looked into the tube, and at first only saw a pink slipper floating at the top. Her eyes drifted down to the shirtless skeleton. His shirt and hoodie had always made him look at least three sizes larger then Frisk, and she gasped at just how small Sans looked. He was still wearing his shorts, and wires were stuck to his skull with rubber.

'What...what's happening?' Frisk asked, and looked at Alphys. Worry beat into her little body and she felt like she might be sick, or cry, or both.

'There are two things to make sure of,' Alphys said, looking at her clip board full of notes. 'The first is to make sure he knows what he wants.' Alphys glanced at Frisk, blushed and looked back at her notes. 'And that was pretty straitforward: he wants to be human. The second is to make sure he has enough magic, which is also true. So...everything is good.' Alphys couldn't erase the look of worry from her face.

'But?' Frisk asked.

'Well...before he fell asleep, he said he needed to wait five years.' Alphys began to sweat. 'This process should have taken only a little bit. Maybe an hour, with Sans's power. I thought it would take less time then that, but he's still…'

Frisk felt her heart fall into her stomach. _Sans told me if we could do anything, it'd have to be five years from now,_ she thought. She looked back at the tube and put her head against the thick glass. 'Bone head,' she said.

'It hasn't even been fifteen minutes yet,' Alphys said with a nervous grin.

'That's right,' Papyrus said. 'This is all experimental, but I have faith in Alphys and Sans!'

Frisk wanted to cry but instead she simply shook her head pitifully.

'Oh, my sweet honey biscuit,' Mettaton said.

'Don't worry,' Alphys said. 'I'll moniter how he's doing. I won't let anything bad happen to him.'

Frisk nodded, encouraged by her friends but still felt uneasy. She scoweled at the floating skeleton. 'I've got a bone to pick with him once he's out.'


	3. Chapter 3: Sans Wakes

undertale: the human equation chapter 3

Days passed and Alphys, alongside Undyne, watched over Sans. Though Frisk grew more and more worried, Alphys said that he was live. She stopped letting Frisk see him on the third week of his long sleep, though, stating that Frisk might be grossed out by the newly forming organs. When Frisk argued, Undyne challenged her to a battle, which Frisk won, but then Papyrus threw a plate of spagetti at her knocking her out. Asgore walked in on the scene, chided everyone, and brought an unconcious frisk to Toriel, who also chided everyone.

Days turned into months, and Frisk's worry only grew with the time that passed. She continued to travel between the surface and the underground to keep relations between monsters and human at ease.

After a year, Alphys gave Frisk the news that Sans's body was complete. He showed no signs of waking, but Alphys let Frisk see him. The tube where Sans slept had become foggy and she could barely make out his face. She tried to see anything she could, but couldn't. When she asked why the tube fogged, Alphys said it was Sans's magic. It was as though he didn't want anyone to see him. Alphys also explained that if she woke Sans before he wanted, it may cause damage to him, so despite Frisk's impatience, she couldn't wake him.

Years passed. Frisk grew up with a hole in her heart, visiting Sans every time she had the chance, in between seeing all of her friends she had made in the underground, and seeing the humans as an embassador.

During that time, Frisk had gotten a house on the surface near the hole separating the underground and the village nearby. She lived with Papyrus, who often went on long drives in his flashy red car. Frisk had a large garden that she often tended to with Toriel. A young man named Oliver had moved nearby, not wanting to live in the village or underground. He was a tall young man with sandy brown hair and olive toned skin.

Oliver had quickly decided to be a body guard for frisk, when one of the humans from the war with monsters tried to assasinate her. It came as a shock to everyone, and the humans in the village were quick to apologize and stated that the elderly man had gone mad in recent years. Even so, seeing an opportunity, Oliver stepped in and said he'd help protect Frisk, for the sake of keeping the peace.

On the fifth eve of Sans's slumber, Frisk sat at her little table in her house with her mother.

Toriel hadn't ever gone back to live with Asgore, but he often accompanied her whens he visited Frisk. He stood outside in the garden watering Frisk's flowers, while the two sat and sipped tea.

 _FIve years._ Frisk stared at her tea, wondering if she should go underground and see Sans. 'Do you think he's going to wake up?' Frisk asked.

Toriel didn't need Frisk to specify that she meant Sans. 'I hope so. Papyrus has seemed awfully lonley since Sans went to sleep.'

'Well, when Mettaton isn't trying to get him to be in a play,' Frisk said with a grin. 'Or when Undyn isn't challenging him to a duel.'

'Sans made up his mind,' Toriel said. 'It was his decision.'

Asgore suddenly let out a hearty greeting, and Toriel's gaze shifted. She watched as Oliver stepped into the garden.

'What if he doesn't wake up?' Toriel asked thoughtfully.

Frisk bit her lip, but she had been thinking the same thing. 'It'd be a shame. I never even got to see him as a human.'

'Haven't you ever wondered what it would be like to...move on?' Toriel asked.

She was being tactful, but Frisk understood what her mother was saying. She crossed her arms and scoffed. 'No one else interests me.' She avoided the sight of Asgore and Oliver having a friendly conversation.

'Oh, really?' Toriel said with a clever grin. 'So you'd rather wait for a lazy bone-head who makes rash decisoins, then persue something new?' Toriel glanced at Oliver again. 'Like, perhaps, a young man that's willing to risk his life both for the sake of you, and for the sake of keeping peace between monsters and humans?'

'For the last time, I'm not going to go out with Oliver,' Frisk moaned.

'I'm not saying that,' Toriel said, waiving her hands. She blushed and sipped her tea. 'I'm just saying, that you haven't really dated anyone, and it might be good to seek out different opportunities.'

'You want grandkids,' Frisk said with a grin.

Toriel looked away and sipped her tea.

The door to the kitchen opened and Oliver stepped inside with Asgore.

'Howdy,' Asgore said with a warm smile. 'Oliver was just telling me about a community garden they're starting in the human village.'

'Oh, that's wonderful,' Toriel said. 'The children at school would love to get involved, I'm sure.'

'That's exactly the plan,' Oliver said, sitting at the table. 'I ran it by the mayor and he got excited. He's really enjoyed working alongside the monsters from underground.' He smiled at Frisk. 'Hullo there.'

Frisk offered a smile and sipped her tea.


	4. Chapter 4: Breaking the ice

undertale: the human equation

chapter 4: I have the guts to break the ice

Sans:

Sans and Frisk walked through Hotland quietly. Sans found himself nervous, and still trying to understand his own body. Just the sight of Frisk was sending different, strange urges coursing through him. Sans had known that humans used their bodies to create other humans, and that it was a normal need but he had no idea how it felt. It was like an itch and he had every temptation to hold her and kiss her and do other things that made him blush and turn away.

Then there was that nagging name: Oliver.

As they neared the ferry Sans finally broke his silence. 'Oliver sounds like a neat fellah,' he said.

Frisk nearly jumped out of her skin.

Sans studied her carefully. 'So, you got a boyfriend,' he said with a shrug.

'What? No, no,' Frisk started.

'Where would you like to go?' the ferryman asked.

'Snowden,' Sans said before Frisk could speak. 'And don't worry, Frisk, it's been five years. I'da been surprised if you stayed single.' _Especially lookin' the way you do,_ he thought.

'Sans, Oliver isn't my boyfriend,' Frisk said, stepping onto the ferryman's boat.

Sans settled in the boat and it set off for his old home. 'Look, you don't need to try and spare anyone's feelings.' Even though he sounded calm and supportive, Sans was incredibly dissapointed. He gave Frisk a hard look. 'You were thirteen back then, Frisk. That's a real young age to decide on matters of the heart.'

Frisk's cheeks reddened and she crossed her arms defiantly. 'You're not listening.'

'Yes I am. If this Oliver kid's not a boyfriend, then what is he?' Sans asked. He struggled to keep the edge from his voice. He didn't expect Frisk to wait for him--or maybe he did, he just wouldn't be mad at her for moving on. After all, he had been gone for five years. _Shit_ , _this is awful_ , he thought to himself.

Frisk put a hand in her hair. 'He-he's a bodyguard.'

'Bodyguard?' Sans tilted his head to one side.

Frisk sighed. 'There was an assasination attempt.'

Sans's heart fell. 'What? When? Are you okay?'

Frisk blinked in surprise. 'Well-yeah, it was a few years ago.' Frisk lowered her head. 'Oliver...he came out of nowhere, really, and asked to protect me. He did it to show that despite the hatred some people have for monsters, there is good, too.'

'Sounds pretty important.' Sans wasn't trying to be sarcastic but he still couldn't keep the edge from his voice.

'Sans,' Frisk started. She reached forward and took one of his pale hands in her tanned ones. 'Please, listen to me.'

Frisk:

'Frisk' came a voice as the ferryman came to a halt.

'We have arrived,' the ferryman said.

Frisk closed her eyes at the sound of the voice. She knew it all too well. She opened her eyes and turned to see Oliver standing on the banks of Snowden. His bright eyes passed from Frisk to Sans.

'Hey,' Sans said.

Frisk stepped lightly fromt he boat to the banks. 'Is Papyrus at home?' she asked.

'Yes,' Oliver said and eyed Sans as he stood beside Frisk. 'But-Frisk, who is this?'

'I'm Sans,' Sans said with a shrug. 'C'mon, Frisk.'

'Wait- _Sans_?' Oliver cried.

'When did Paps move in with you?' Sans asked. 'Where do you live?'

'I didn't want to move too far away from the Underground,' Frisk told him. 'So, everyone helped me build one near the hole I fell through when I was a kid. We even made a stairwell leading up to the surface. Alphys is working on an elevator.' Frisk had so much she wanted to ask, but she simply couldn't. She wanted to demand why Sans had wanted to be human at all, but how could she? He seemed happy and healthy enough. 'Papyrus never said he was lonely, but we could tell. And he kept going to Undyne's house at three in the morning with spagetti telling her that he couldn't eat it all by himself.'

'Yeah,' Sans said guiltily, 'that's no good.'

Frisk and Sans and Oliver climbed the stairwell to the surface. Frisk told him about the humans and the monsters and how things had become peaceful, but the humans needed to speak with the rulers of the land about what was happening. She didn't quite know what that would mean, but she would protect her friends regardless.

By the time they were inside of Frisk's house, it was night time. Sans paused and stared at the sky. Frisk did, too, but she had run out of words and could think of no more small talk.

'What're you doing?' Oliver asked.

'Its just...I didn't get to see the night sky before I fell asleep,' Sans admitted. He glanced and Frisk and grinned.

Her heart went aflutter in her chest and she quickly turned away, willing herself to calm down. _All he did was grin_ , she thought. _That's no different then he used to act. Get ahold of yourself!_

Together they stepped into Frisk's house and she sourly wished that Oliver would go away. Normally he went home when the sun hit the horizon.

Frisk stepped lightly through the backdoor and into her kitchen. 'Paps?' she called, hurrying deeper into her home. She was thankful to get away from the men, if only for a minute.

'Human! I have just discovered that they came out with a new model of the car I drive!' Papyrus lumbered into the hallway. 'It's big enough to carry everyone, too! We could all go on a trip to the human cities!'

'Papyrus,' Frisk said breathlessly.

'Ah, Frisk.' Papyrus took Frisk's shoulders in his boney, gloved hands. 'Your face is red and you are out of breath. Have you been watching anime with Alphys again?'

'No, I,' Frisk started.

'Aha! So you've been training with Undyne,' Papyrus said. 'That must have worked up your appetite. Let me make you the most delicious sandwich.' Papyrus stepped into the kitchen, and Frisk followed him.

Sans simply grinned and shoved his hands in his pockets as his brother stepped onto the tile. Oliver studied everything closely, and skeptically.

Papyrus stared at Sans before tilting his head to one side. 'Wowie, you're dressed just like my brother used to! Who are you?'

'Hey, bro,' Sans said.

A flicker of orange magic briefly shimmered in Papyrus's eye socket. 'Sans?' he asked.

Sans nodded and ran a hand through his white hair. 'Sorry I didn't have the guts to see you before now.'

Papyrus gasped and put his hands to his face. 'I thought-we all thought you might not wake up!' His joy quickly turned to anger. 'Do you have any idea all the trouble I've gone through for you? I did not miss you, not one single bit!'

'I'm pretty hard to miss,' Sans said, still grinning.

'Your pet rock has been thrown at my door multiple times and every time I feed it sprinkles it just sits there!' Papyrus cried. 'The swirling vortex of garbage in your room collapsed and now you're room's a mess, and I haven't been able to clean it for five years! And our house-I couldn't-now I,' Papyrus stopped.

Sans teleported forward and put his arms around his brother. Papyrus looked as though he didn't know what to do for a moment. He put his arms around Sans.

'I didn't miss you at all,' Papyrus said. 'I was strong and I helped the human.'

'I know, Paps,' Sans said. 'I'm sorry I was gone for so long.'

'So...this is all real?'

Everyone froze and looked at Oliver, who still stood by the door, staring incredulously.

'What do you mean?' Frisk asked.

Oliver ran a hand through his dark hair. 'I thought Papyrus made Sans up and we were all just being nice.'

Everyone went awkwardly silent again, and Frisk's stomach was doing nervous summersaults.

'Frisk, c'mere for a second,' Oliver said, taking Frisk's arm.

'Wait,' Frisk started, but Oliver ignored her. As he pulled her out of the house she was aware of Sans's gaze firmly planted on her. _Jesus, this does not look good,_ she thought.

Oliver pulled Frisk into the garden and let her go. He ran his hand through his hair and paced under the silvery moonlight.

'Oliver, what's the problem?' Frisk asked. 'He's an old and dear friend.'

'I've never met him,' Oliver started, and put his hands on his hips. 'Toriel mentioned him once or twice but I didn't really think about it at the time.' He studied Frisk carefully. 'She seemed really mad at him.'

Frisk laughed and shook her head. 'She was.'

'Why?'

Frisk frowned. 'Well...before Sans decided to try and become human, I sort of...told him I had feelings for him.' She couldn't look Oliver's way, because she knew he'd be disappointed. Frisk had never wanted to be Oliver's girlfriend, but she had always suspected that he wanted more.

'You were thirteen back then,' Oliver said. 'He was taking advantage of you.'

Irritation swept through Frisk. 'He left for five years,' she said with a hollow laugh. 'He...never answered to my feelings.' She looked at Oliver angrily. 'He said that we couldn't do anything and that I was too young.'

Oliver shook his head and looked at the ground. 'Frisk...just...be careful.'

Frisk was still irritated but she couldn't help but smile. 'I don't need to be careful around Sans. He doesn't want to hurt me.'

Oliver scoffed and crossed his arms.

'Hey,' Oliver suddenly said. 'Tomorrow, I'd like to take you somewhere.'

Frisk's mind went blank for a moment. 'Well, I,' she started, trying to think of some excuse.

'No excuses,' Oliver said with a smile. 'You're going out on the town with me, and that's that.'

Frisk sighed. _Well, I have to let him down at some point, don't I?_ 'Where?' she asked.

Oliver simply smiled. 'It'll be a surprise. I'll come around in the morning. Until then…' Oliver glanced inside. 'Don't be alone with him.'

Frisk wanted to argue again but Oliver turned on his heel to leave. Frisk watched him go and lowered her head. _I do not want to go out with him tomorrow, she thought,_ stepping inside. _Sans just woke up._

As she stepped into her house, she heard Papyrus excitedly say: 'And it'll be so wonderful, and you can live here with us!' He was squeezing Sans and grinning like an idiot.

Frisk froze when she stepped through the door.

Sans looked her way and offered a cheesy grin. 'So...I guess I'll live here, then.'

Frisk stared at the two of them and as Papyrus let out a: 'YAAAAAAAAY!' He let go of Sans, babbling about cars and humans while he ran into the living room.

'So. What'd prince charming have to say?' Sans asked.

Frisk couldn't help but let out a groan and she put her face in her hands.

'Sounds good so far,' Sans said.

'He doesn't trust you and he wants to take me out tomorrow,' Frisk said.

'Not like a boyfriend at all,' Sans said cooly.

'Bodyguard,' Frisk said.

'Frisk, it's okay,' Sans told her.

Irritation filled Frisk again. She stepped forward earnestly. 'Sans, you don't understand,' she said. The sight of him, trying to tell her that she should date someone else, filled Frisk with determination. 'I never,' she started but Papyrus interupted her.

'Sans! The humans invented this can of what's called silly string! Come, look!' he cried in delight.

Sans stared at Frisk for several seconds before saying: 'I'll be right there, Paps.' Then he turned to Frisk and gently touched her shoulders. 'We're gonna have to save this conversation after your date.' He turned quickly but Frisk stamped her foot.

'It isn't a date!' she cried.


	5. Chapter 5: The Date

undertale: the human equation

-Just going to remind you guys that I don't really hold back with my writing. This is what you'd call a 'trigger warning'. If even slight sexual aggression without consent makes you uneasy or makes you relive something atrocious that happened to you, nothing sexual happens here but there is aggressive conduct that takes place. I understand that things like this can make one uneasy. But, at the same time, we all know that Sans would probably tear apart anyone that tried to hurt Frisk. I just thought I'd be nice and let everyone know. Enjoy the read.

chapter 5: The Date

Frisk:

Morning came and the smell of food wafted into Frisk's room. Normally, Toriel or Asgore would stop by and one of them would make coffee. Asgore liked to make pancakes, too. As Frisk struggled to open her eyes, she wondered what they would say if they knew Sans was awake. Alphys had probably told them already. Toriel would probably be disappointed that Sans was living with her. Despite her mother's support over the years, she had harbored an unspoken anger at Sans that Frisk didn't understand.

Frisk's eyes suddenly snapped open and she sat upright. _Oh, crap, that's right, he's here!_ She quickly threw on a large overshirt and slacks and ran into the hall. The smell of bacon and eggs rolled througout the house and she hurried into the kitchen.

Sans had an open book floating by his head, using his blue magic. He was making bacon and eggs and humming a little as he did so. The crackle of grease in the pan had muffled Frisk's footsteps, but she still stared until he turned around.

Sans chuckled. 'You should go look in a mirror, your hair's a mess.'

Frisk put a hand in her hair, but didn't care. Sans had known Frisk when she was twelve, and back then she hadn't been sure if she was a girl or a boy. She couldn't imagine that she looked very good back then, when she hardly cared whether or not she brushed her hair at all, let alone if it looked like it belonged to a girl or boy.

'You're still here,' she said without thinking.

'Did you want me to leave?' Sans asked turning back to his food.

'I thought it might have been a dream,' she said and slowly sat at the table.

'Nope. I'm as real as these eggs,' he said. 'Which, by the way, do you smell that? It's incredible.' He moved his hand and lowered a gigantic plate of eggs and bacon onto the table. He sat down across from Frisk and began to eat. 'I can't believe how fun it is to eat. How do you not eat everything in sight?'

Frisk laughed and shook her head. 'Sometimes I do,' she said.

'Well, eat up, I know I'm pudgy but this isn't all for me,' Sans said.

Frisk excitedly began scooping food onto a plate.

As she started to eat, Sans said: 'So. Dream about me often?'

Frisk nearly choked on her eggs. She _had_ dreamt of him often, and those dreams ranged from memories of playing together to other, weird sexual fantasies that made her blush and chug water to keep from choking. 'Sometimes,' she said breathlessly.

'Frisk?' came Toriel's sweet voice. 'I brought some pie, I thought,' Toriel paused once she was in the door and saw Sans.

Sans stared for several moments, slowly chewing his food. 'Hey, Toriel,' he said with a waive.

Toriel's eyes narrowed. 'Sans,' she said evenly. She gave an evil grin. 'I have had a bone to pick with you for five years.'

'Can it wait until after breakfast? This is my first time eating bacon and it's one of the greatest things I have ever tasted,' Sans replied.

Toriel sighed. 'Frisk, sweetheart, I brought some pie but it's for Asgore.' She had been holding a pie the entire time, and Frisk half expected her to throw it at Sans.

'Ooooh, so you're baking him pie now?' Frisk asked, raising her eyebrows.

Toriel gave her daughter a hard look before putting the large pastery in the fridge. 'Don't think anything of it. It's a gesture of goodwill.'

Frisk chuckled, which earned her another hard look.

'I know what you're all trying to do,' Toriel said and sat down. 'And I wanted you to know it won't work.'

'Well, you know he still loves you, right?' Frisk asked.

Toriel jumped and scratched her neck. 'I know,' she said.

'So what's the big deal?' Frisk asked curiously.

Toriel hesitated before saying: 'What happened was...unforgivable.'

'Was it?' Frisk pressed. She hadn't pressed Toriel so much about Asgore before, but the two were in love. One of them just needed to bend and express themselves.

'Yes,' Toriel said stubbornly.

'How is old fluffy buns anyway?' Sans asked.

'Stubborn,' Toriel replied.

'Cute,' Frisk said. Toriel glared and Frisk rolled her eyes.

'You've gotten so brave in just one night,' Toriel remarked. She then glared at Sans. 'I do hope nothing out of the ordinary happened to inspire that.'

'It's got nothing to do with me,' Sans said, holding his hands up. 'I'm here for Paps. And she's got a date as soon as Olive-head gets here.'

'Oliver,' Frisk corrected.

'A date?' Toriel cried.

'It's not a date,' Frisk started.

Toriel looked from Frisk to Sans several times. 'But...you're going out with him today?'

'It isn't what you're thinking,' Frisk said. She didn't have the courage to tell Toriel that she only agreed to Oliver's plans to let him down.

'Quite scandelous indeed!' Papyrus boomed. He wore a nightcap and put his hands on his hips. 'I do suspect that miss Frisk here has motives beyond our understanding!'

'What are you talking about?' Toriel asked.

'Eggs!' Papyrus cried.

'Made 'em myself, eat up, Paps,' Sans said.

'Although I have no tongue, I will do so with vigor!' Papyrus said and sat down at the table.

Although Toriel continued casting dark glances in Sans's direction, breakfast turned out quite well. Sans's appearance definately made things livlier and Frisk was thankful to have him there. When someone knocked and the backdoor opened Frisk's heart fell.

 _Please be Asgore, please be Asgore,_ she thought.

Asgore stepped in with a wide smile. 'Howdy everyone.' His gaze fell on Sans. 'Well, now, who do we have here?'

Just as relief flooded Frisk, Oliver stepped in just behind her father. Frisk's heart fell.

'Hello everyone,' Oliver said lightly.

'Oliver! Sans made eggs!' Papyrus said. 'Although I could have made them better.'

'Gotta get a tongue first, bro,' Sans said with a grin.

'Yes! That shall be my goal today. Toriel, where might one find a tongue? I'm asking for a friend,' Papyrus said.

'Oh, um,' Toriel said, but she seemed flustered by the appearance of Asgore.

'Frisk,' Oliver said. 'If it's alright with you, I'd like to leave now.'

Frisk hesitated and looked around hoping that someone would speak up and keep her from going out with Oliver.

'You gave your word,' Sans said. He was still grinning, but something in his voice sent shivers up and down Frisk's spine.

 _He sounds mad,_ Frisk thought. _Maybe because I promised I'd wait for him._ Frisk cleared her throat and stood. 'Alright,' she said to Oliver. _But why would he be mad?_ Frisk followed Oliver into the garden. _It's not like we were ever together._ Oliver turned and smiled in the morning sunlight. _I was never even sure if he had feelings for me._

'You look beautiful,' Oliver said with a dashing smile.

 _Why did Sans choose to become human?_ The thought burned its self into Frisk's heart, but she willed herself to smile. 'I haven't even brushed my hair yet.'

'Then we should take you to a salon and get you a haircut,' Oliver said proudly.

'What?' Frisk asked.

Sans:

Watching Frisk leave with Oliver was gut wrenching. Oliver even smiled smugly and stuck out his chin at Sans before shutting the door. Sans couldn't help but glare, but he knew it was useless. He had no right to be jealous. He had _told_ Frisk to go out and date people. Still, some small, selfish part of himself had hoped against hope that maybe she'd say she had waited five long years for him to wake up.

'Sans,' Toriel said sternly.

Sans jumped. He smiled lovingly. 'Yes, ma?'

Toriel looked like she could set fire with the anger in her gaze. 'You knew I'd be angry.'

'Whatever for?' Sans asked, but he knew what was going on.

Asgore sat down and began eating, watching his wife fearfully.

'Did you make a move on my daughter when she was a child?' Toriel growled.

Asgore coughed and suddenly he got angry, too. 'You _what?_ '

'Hey, hey, hey, you've got it all wrong,' Sans said.

'Then what is all this about?' Toriel demanded. 'Did you think you could just become a human, waltz in here, and take advantage of Frisk?'

'Well, no, that's not what I want at all,' Sans stammered. He was genuinely afraid that Toriel might kill him.

'Then why did you do this?' Toriel demanded.

Asgore's gaze softened. 'Goodness,' he said, 'you really fell in love with her back then.'

Sans couldn't speak for a moment. He stared at his king and queen and laughed without thinking. He put a hand in his white hair and shook his head. 'I...I told her we couldn't do anything,' he said weakly.

'Well, not necessarily,' Asgore started but Toriel shot him a dark look.

'Oh, yes, you couldn't _necessarily_ do anything,' she said haughtily. Then, she, too softened. 'She was thirteen.'

'I know,' Sans said, trying to catch his breath. He sighed. 'Listen, Tori, I didn't want to hurt her or take advantage of her. That was never my intention. And yeah, I thought becoming human would take like an hour or two. Not...not five years. I wanted to help raise her right. I mean,' Sans let out another nervous laugh. 'She saved us.'

Toriel and Asgore nodded in agreement.

'And...if that guy is the thing that makes her happy, I'm not gonna get in the way,' Sans told them.

Toriel suddenly looked very worried. 'You really mean that?'

Sans nodded. 'I just want her to be happy,' Sans said finally.

'Oh, well then you should come with Mettaton and I to spy on their date,' Papyrus said. 'Mettaton said that we needed to make sure Oliver doesn't do anything suspicious and that Alphys wants to make sure Frisk is happy.'

Everyone stared at Papyrus for a moment.

'That reminds me! I'm supposed to meet Mettaton at the train station! I'll see everyone later!' Papyrus suddenly ran from the table and jumped out the window, shattering the glass.

Sans stared at his bony brother running through the garden. _Shit._ 'I'm gonna go make sure he doesn't mess anything up for her.' His heart was twisting in knots. Seeing Frisk on a date with someone else was the last thing he wanted to do, but if he knew his brother, he knew Paps would find trouble.

'Sans,' Asgore suddenly said.

Sans paused at the door.

'I think Toriel is overreacting,' he said flatly.

'What?' Toriel demanded.

Asgore put his giant paw on hers and she blushed and quieted.

'If you love her, you should persue her with all you've got,' Asgore said. 'Because even if it ends in heartache, you can at least say that you fought hard till the bitter end.'

Toriel stared at her husband's hand on hers.

Sans grinned. 'Talk's not worth much if her heart's set on someone else,' he replied grimly and stepped out of the house.

Sans teleported just behind his brother, who was standing behind a bush by the train station. He couldn't blame Toriel for being mad. When Sans realized he loved Frisk, he had been mad at himself. What sort of gross wierdo falls in love with a kid? That was despicable. That was why he had to push her away. That was why he told her to wait, and why it wasn't fair for him to be jealous. It was selfish. Selfish Sans.

'Paps,' Sans said.

'Brother! Take a look,' he said. Papyrus was oddly well hidden and Sans crouched beside him.

'Hey, we should head back,' Sans said.

'Nonsense! Oliver must live up to our expectations if he's to be with Frisk,' Papyrus said. 'Now, watch with me! We are the heros, we must protect our princess!'

Sans begrudgingly looked through the bushes.

Oliver and Frisk were sitting at a table and sipping on something to drink. Frisk's hair was slightly shorter and neatly cropped. _Haircut,_ Sans thought. Though Oliver looked like he was having the time of his life, Frisk looked-

'Absolutely terrible,' Mettaton's voice came in a whisper.

Sans nearly jumped, and his magic reacted to his surprise.

'Oh, Sans, you're hot to touch today,' Mettaton said with a giggle.

'Guys, seriously, this is weird and not at all what Frisk would want,' Sans said, trying to calm his thudding heart.

'Well, if you want to be lazy about it, then you can go back and hang out with Asgore and Toriel,' Papyrus said haughtily. 'Honestly, though, they've gotten so lovey dovey that it would make me sick to my stomach...if I had one.'

'Besides, Alphys doesn't really trust Oliver,' Mettaton said.

'Why?' Sans couldn't help but ask.

'She says he reminds her of an anime character that tried to steal away the main character's heart and ends up becoming dangerously jealous,' Mettaton stated calmly. 'Also, I'm pretty sure that he was behind the whole bombing incident, but that doesn't matter. We want to make sure that he makes her happy!'

'What?' Sans cried.

'Shh! Sans, do you want to get caught?' Papyrus asked.

'Oh, it's scandelous, darling,' Mettaton said. 'That boy, Oliver, is the son of the man that tried to hurt Frisk. On the day that he asked to be her bodyguard, he had residue from the bomb all over him but...it's just as well, he hasn't done anything since then. And there's no proof that he did anything.'

'But, what if he tries to hurt her?' Sans demanded.

'Why do you think we're here?' Mettaton asked raising an inquisitive metal brow.

 _I thought Paps was just being ridiculous._ Sans turned to Frisk. _But they think this guy might mean trouble._

The couple moved from the coffee shop to a line of shops in the downtown district. Some sort of summer festival was going on, so there were vendors and kids in costumes everywhere. Luckily, Mettaton and Papyrus fit in just fine. Sans couldn't keep his eyes off of Frisk, who looked extremely uncomfortable.

 _Would I make her look that uncomfortable?_ He thought. _If I took her out, what expression would she make? Is it even alright for me to want that?_

Papyrus and Mettaton got lost playing a game at one of the vendor's stands. Sans continued to follow Frisk, feeling miserable and creepy. As the sun set, Oliver took her to a little secluded spot by a lake. Sans was about to take his leave, when he heard Frisk say:

'You know-it's so pretty, I don't want to leave this spot.'

Something about the tone of her voice made Sans turn. She wasn't afraid, but there was an edge, like she was worried. Sans was hiding behind a tree, and they were just outside of the main street where all of the festivities were going on.

'Oh. Alright,' Oliver said lightly.

 _I should leave, and go find Paps,_ Sans thought.

'Frisk,' Oliver said gently. 'There's something I wanted to talk to you about.'

 _Here we go,_ Sans thought.

'What is it?' Frisk sounded expectant. She sounded worried. Sans was becoming uneasy, too.

'Frisk...my father did a terrible thing. He tried to hurt you.' He took a deep breath. 'But...I've been thankful every since.'

Sans's breath caught in his throat. It was another strange feeling that he only seemed to get around Frisk. When he dared to peek from his hiding spot, he saw Oliver staring at the sky. Frisk had squeezed her eyes shut, as though she were in pain. _Why?_ He thought. _Shouldn't you be happy?_

'I wanted to tell you yesterday,' Oliver said. 'See...it's sort of a celebration. For Asgore and Toriel. He's confessing his love for her. He chickened out yesterday, and he's going to tell her today.'

Frisk opened her eyes and clenched her fists.

'So I thought it only best that, to support him, I own up to my own feelings, too.' Oliver stood proudly.

Sans hated him. He hated the chisled cheeks and the day old stubble. He hated how fit the man was-why couldn't Sans be that fit? He _wanted_ to be thin, but it just wasn't how he saw himself. Sans touched his stomach and again he wanted to leave. Frisk probably hadn't given a second thought about Sans. Even if Sans was an old friend that had helped Frisk fight a demon with Asriel's name, he wasn't much compared to the disgustingly beautiful man standing with Frisk at dusk.

 _I shouldn't have become human,_ he thought. _It was a dumb mistake. Maybe Alphys can turn me back._

'I'm in love with you, Frisk,' Oliver said. 'And I want us to be together.'

There it was, like a knife in Sans's heart. He clutched his chest and leaned against the tree. _Good god, is this what Asgore felt like when Toriel left him?_ It almost made sense that Asgore had secluded himself when Toriel left. The pain in Sans's chest was enough to drive anyone out of their mind.

Frisk took a deep breath. 'I'm sorry,' she said. 'I don't feel the same.'

The sun sank beneath the horizon. Thousands of stars glimmered with the moon overhead. The green grass on the hill swayed in a cool breeze blowing in from the lake. Sans lifted his head and stared in shock. Oliver did the same.

'What?' Oliver said with a hollow laugh.

Frisk was determined. 'I don't feel the same way. I'm sorry.'

Oliver laughed again. 'Are you kidding me?'

'I'm sorry,' Frisk said again. She glanced at the festival and then back at Oliver.

'I've been protecting you for three years,' he said, putting his hand in his hair.

'And...I've appreciated it, I like our friendship,' Frisk started.

'I don't want a _friendship_ ,' Oliver snapped.

Frisk stepped back.

Oliver's eyes looked wild. 'You owe me, Frisk.'

'I don't,' she started, but Oliver grabbed Frisk's arm.

'You do,' he snarled. He grabbed the back of her head and forced her to kiss him.

Anger erupted in Sans's chest. He suddenly held up his hand. Blue, magical flames burst from his hand and the same magic light surroudned Oliver. He slammed Oliver into the lake and hid behind his tree, panting. _How dare he?_ His thoughts demanded. He put a hand in his hair.

Frisk stumbled back and fell onto the grass. She looked around herself. 'S-Sans?' Frisk called.

Sans panicked, and teleported away.

He found himself in the mainstreet, where vendors had hung little paper lanterns on strings. Sans dizzily stared at the warm lights and listened to the vendors shouting about their goods for a few solid minutes. Sans teleported again, making his way back to Frisk's house. He burst inside and stumbled into one of the bedrooms. He collapsed onto his bother's race car bed, his heart beating wildly.

 _Shit,_ he thought. _She knows I was there. Shit. Paps. Shit shit shit!_

Sans put a hand on his head. He was reeling from his anger at Oliver, and the excitment he felt when Frisk denied him. His whole body felt like it was on fire.

 _What do I do? This is her house, we're gonna see each other at some point._ Sans closed his eyes and pretended to be asleep.

He heard Papyrus and Mettaton come home, speaking loudly and remarking at all the fun things they had done while at the festival. Mettaton made Papyrus happy, which pleased Sans. Then he heard Frisk's voice and his heart began beating wildly.

'Calm down,' he whispered to himself.

'I just don't know what happened to Sans, though,' Papyrus admitted. 'I'm sorry, Frisk, I'd tell you if I knew.' Papyrus opened the door to his room and Sans's stomach sumersaulted. 'Oh!' Papyrus cried. He suddenly turned and whispered: 'Frisk, keep it down, he's sleeping.'

 _Frisk is right there, staring at my back, she called my name, she doesn't have feelings for Oliver and I ran away after throwing him in a lake!_ Sans gripped the sheets of his brother's bed.

'Papyrus, darling, we've got to do the unboxing show with all the goodies we've won!' Mettaton called. 'The public simply cannot wait!

'Right! I, the great Papyrus will show the masses just how great I am with all of the amazing things I've won at the summer festival!' he cried, stomping through the house. He had left the door open and Frisk's shadow stood stock still. She stepped forward, and Sans thought his heart might leap into his throat.

Frisk stopped right beside the bed and sat on it. She leaned forward and put her head in her hands. Sans was rigid, unable to move. When he heard a sniffle, he turned.

She was crying.

'Kid,' Sans said.

She blinked in surprise and suddenly wiped her face. 'Oh,' she said, 'I thought you were asleep.'

Sans put his hand on her shoulder. He offered a kind chuckle and said: 'Date couldn't have gone that bad, huh?'

Frisk's lip trembled.

Sans wiped away one of the tear streaks. He felt like his heart was doing summersaults in his chest. 'C'mere, kid,' he said and pulled her close.

Frisk sniffled and leaned on his chest.

'You don't have to say nothin',' he told her. He ran his fingers through her freshly cut hair.

'I've lost a friend,' Frisk told him.

Sans gulped at the lump in his throat. 'Oh?'

Frisk suddenly moved away from Sans. She studied his face intently, wiping at her tears.

'What?' Sans asked nervously.

Frisk closed her eyes and shook her head. 'I'm sorry,' she told him. 'I should go and make sure Mettaton doesn't make my kitchen explode.' Frisk moved away from the bed and made for the door.

Sans felt frozen, and he wasn't sure if he had helped or hindered. 'Frisk,' he started.

She turned, looking expectant.

'If...you need me,' he said lightly.

'I'll wake you,' Frisk said lightly and stepped away.'

After she left Sans collapsed in Papyrus's bed. _Shit._


	6. Chapter 6: One fish, two fish

undertale: the human equation

chapter 6: One fish, two fish, love fish, loo fish

Sans:

It had been a week since the date. Oliver hadn't shown up since, although Toriel and Asgore made a habit to show up every morning. Sans enjoyed their company, and so did Frisk. At first, they asked for the whereabouts of Oliver, who had simply stopped showing up. Frisk told them he fell into a lake and came out angry and hadn't spoken a word to her since. Sans couldn't help but snicker, and when Toriel rose a quesionting brow, he quieted.

When Oliver finally did show up, it was when Frisk was heading out to a meeting at the town hall. He glared at Sans, and walked with Frisk towards town. After several moments of staring, Asgore decided to follow, both wanting to see Frisk and the townspeople.

Sans and Paps often hung out. Undyne was still teaching Papyrus to cook, which was always entertaining, especially when Alphys joined and got mushy with her girlfriend. Sans was adjusting to his new life on the surface, with a body. He got hungry a lot, and everything tasted so good. At first, he wanted to cook and eat everything, even the weird stuff that Undyne and Paps cooked. Then, after a horrible stomach ache, Toriel instructed Sans to eat three meals a day instead of, well, everything.

Frisk rarely ate, and Sans didn't understand it. He tried to talk to her about it, but she only offered one word responses and made some excuse to run away. At first, Sans thougth: _Well, a big thing just happened maybe she needs space._ As the days went on, though, he noticed she didn't want to talk, she didn't want to hang out alone with him, and sometimes she wouldn't even look at him.

She was avoiding him.

Sans wanted to demand why, but he knew better. Maybe it wasn't just him, maybe it was all men, he told himself. Sometimes when people had experiences like what had happened to her, they secluded themselves. Only, he had seen Frisk alone in the Garden with Paps. Then he thought: why? Why didn't she want to see him? Sans had taken over one of the bedrooms in the house and spent a good day trying to figure it all out. He thought about all of the different timelines he knew of, wondering if he could alter something, (though he knew he couldn't). He was half tempted to find Flowey, but almost as soon as he had that thought he chided himself. Flowey was impulsive and malicious sometimes. Working with him wouldn't be a good idea.

Then it dawned on him: she knew. She had to. She knew that Sans had flung Oliver into a lake like a rag doll. She knew, and she was angry.

 _Shit. I gotta talk to her._ The thought sent feelings coursing through Sans, and he did his best to suppress all of them. What good would the desire to kiss her do? What good would it do to hold her? She had _let_ him hold her, though. She had even cried. Why did she want nothing to do with him, all of a sudden?

Finally, one weekend morning Toriel and Asgore were wandering through the garden, and Papyrus was out in town with Mettaton, and Frisk was alone in the kitchen. Sans had only just woken, and froze when he saw her. The very sight of her sent his heart rattling against his bones, but he reminded himself to be strong.

'Hey, kid,' he said and sat down.

Frisk jumped and blinked in surprise. 'Hello,' she offered. 'You normally aren't awake this early.'

'I know,' Sans groaned. 'Toriel's got me doing this...three meals a day thing, and it's got me waking up at weird times.' They quieted and Frisk fiddled with her shirt.

'I can go ask her if there's any pie left,' she suddenly said and stood.

'Wait,' Sans said.

Frisk froze and turned to him.

She looked uncertain, and it made Sans uneasy, too. _How do I fix this?_ Sans sighed and sat back. 'Did you hear about the guy who got his entire left side cut off?'

Frisk's eyes widened. 'What? No! How did that happen, is he okay?'

'Yeah,' Sans said. 'He's _all right_ now.'

Frisk's face went blank. She snorted. 'Sans, that was bad, even for you.'

'It's a lot better then the book on anti gravity I picked up,' Sans said with a grin. 'I could hardly put it down.'

Frisk burst out laughing.

'Did you hear the one about the girl that started avoiding her friend?' Sans asked.

Frisk's laughter quieted. She shook her head.

'I did, but I didn't get the punch line,' Sans told her. 'What do you think it was?'

Frisk sat down slowly. 'I don't think that joke was very funny.'

Sans shrugged. 'Are any of my jokes funny?'

Frisk let out an exhasparated sigh. 'I'm sorry,' she told him.

'Why?' Sans asked. 'You didn't do nothin'.'

She looked at him then with her large, doe-like eyes and Sans's heart melted. He kept his expression calm, but she had no control over hers. 'Oliver kissed me,' she said and buried her face in her hands.

Sans stared at her as she peeked through her fingers.

'Oh?' he asked.

'I didn't want him to,' she started. She lowered her hands. 'I...rejected him. He forced it on me.'

Sans couldn't believe what he was hearing. For the last week he had become certain that he had done something wrong.

'It isn't what you think,' Frisk said. 'And I think maybe I've got some kind of magic powers. Because, when it happened, a burst of blue light flung him into the lake.' Frisk studied him carefully, as though daring him to tell her any different.

Sans was still staring in shock, trying to piece together what she was telling him.

'It was blue,' she told him. 'Blue fire, just like…'

 _Say it,_ Sans thought.

Frisk merely stared. 'I felt like I was being watched,' she said. She blushed and put her hand on her chest.

 _Shit._ Sans ran a hand through his hair and sighed. 'Alphys didn't trust Oliver,' Sans said. 'She sent out Mettaton to watch you. Then he got Paps involved, and Paps got me involved. They got distracted at the festival, but, em…' Sans scratched his head.

'So, it was you,' Frisk finished for him.

Sans wouldn't look her way. He couldn't. 'I had an awful feeling in my gut,' he admitted. 'I was gonna turn back and go find Paps, but then Oliver did what he did and I couldn't just do nothin' while he forced you to kiss him.' Sans frowned staring at his hands on the table. After a few horrible seconds of silence, he said: 'Look, I'm sorry. I shouldn't have interfered.'

'If you hadn't, what do you think he might have done?'

Her voice was sharp, but Sans rose his gaze to find her full of determination.

'He said I owed him,' she said. 'Do you really think he meant only a kiss?'

Sans didn't think Oliver only wanted a kiss, and he hadn't realized that Frisk, too, understood the implications.

'He was my friend,' she said. She looked down. 'I trusted him. But, I never…'

'S'all right, Frisk,' Sans said.

'I never stopped thinking about you,' she told him. 'I never…'

A blue spear broke the window and landed on the table between the two.

'Ack!' came Alphys's voice.

 _Dammit,_ Sans thought. _Just when things were getting good._

'S-sorry, Undyne and Asgore are batteling,' Alphys called. 'Are you two alright?'

'Haha! Got you now fluffy buns!' Undyne yelled.

'Wowie, will you teach me to do that?' Papyrus cried.

Sans sighed and put his hand in his hair.

'We're fine,' Frisk called. She stood, about to leave, but Sans couldn't stop himself.

He stood and grabbed hold of her hand. She froze and stared at him with wide eyes. 'Frisk,' he said. 'Would it have been different if it had been me instead of him?'

Her cheeks had turned red and her gaze was soft. What was that, happiness? Was that what humans looked like when they were happy? _Am I making her happy?_ Sans suddenly let go of her hand, remembering how uncomfortable Frisk had looked in Oliver's company.

The door suddenly opened. 'Frisk,' Alphys said. 'Y-you should come and see!'

'Um,' Frisk said, 'yes.'

'You, too, Sans,' Alphys said with a smile.

Frisk glanced at Sans, and headed outside. Sans stood still for a long moment. He stepped through the doorway and into the garden. Asgore and Undyne were sparring in the grassy feild beside Frisk's house. Sans watched and shoved his hands in his pockets.

 _What a mess,_ he thought.

Frisk:

Frisk could not sleep. Papyrus had gone to bed to the sound of Sans reading him a story. The walls between his and Frisk's room were thin, and she could hear the sound of Sans's voice from the other room. The sound sent shivers through her body and she lay in her bed, trying to piece together what the last week had meant.

She had suspected that Sans had been watching her while she went out with Oliver. The bluefire was his magic's telltale giveaway, and Frisk had chosen her words carefully so that he might admit it. Not only had Sans watched, but he had protected her and even given her space when he thougtht he might've gone too far. At least, that's what Frisk could gather from her own speculations.

When Papyrus started snoring, Sans quieted and Frisk could hear him leave the room. Frisk touched her hand, where Sans had grabbed her.

Oliver had forced Frisk into a kiss that she didn't want. She remembered cringing at his touch, not only from his hand on her arm, but his lips against hers. Sans's question burned within Frisk: _Would it have been different if it had been me?_ What if Sans had been the one to kiss her? Frisk buried her face in her hands. She would have loved it. She had been wondering since she first laid eyes on him in Alphys's lab just how soft his lips were. But how could she say that?

After hours of wrestling with her own thoughts, she stepped from her room and went into the kitchen to get a glass of water.

 _I need to think rationally about all this,_ Frisk thought. _Sans and I aren't an item. We are not a couple. We never have been. I don't even know if he has feelings for me._ Then why had he followed her around on her date? Well, the answer was easy: Paps. Sans would do anything for Papyrus. It was clear that he felt guilty about being asleep for so long. And if Paps was about to do something obnoxious, like interfere someone's date, he would try and keep him out of trouble. Sans had been looking out for Papyrus. _But he flung Oliver into a lake. Right after Oliver kissed you, before he could do anything else, Sans flung him into the lake!_ But Oliver was going to force Frisk into whatever he wanted. Frisk cringed.

'What're you doing?'

Frisk jumped and turned to see Sans in the kitchen.

'You left the water runnin',' he said.

Frisk turned to the sink and reached for the water knobbs. Before she even touched them, a blueish light radiated and they turned themselves off.

'I got it,' Sans said. He tilted his head to one side. 'You're normally asleep by now.'

'I couldn't sleep,' she told him. 'I was getting water,' she said.

Sans eyed the empty cup in Frisk's hand. She put it down and offered a smile.

'Somethin' on your mind?' he asked.

Frisk stared at him. _You,_ she thought, but she shook her head.

Sans chuckled.

'What?' Frisk asked.

'C'mon, kid,' he said. 'Don't lie to me.' His gaze flickered with a sense of knowing. 'You can fool your parents with simple talk, but you won't fool me.'

Frisk calmed down a little and leaned on the counter behind her. 'Well. _You're_ on my mind.'

Sans's brow rose.

'Why?' Frisk started. 'I keep coming back to that question.' She grinned, trying to appear nonchalant, but she was pouring her heart out and she couldn't stop. 'Why would you ask if things would be different if _you_ had been in Oliver's place? Why did you fling Oliver into a lake?' Frisk paused. 'Why did you decide to become human?'

Sans's cheeks turned red and he rubbed his jaw. He stepped forward until he was much closer to Frisk and leaned on the counter beside her. The closeness set fires in her body that she didn't quite understand. Had it not been for her incredible nervousness, she would have thrown her arms around him and kissed him and done other things that made Frisk blush and shake her head. Sans crossed his arms and rested them on his belly.

'I was curious,' he told her. 'Paps and Mettaton were intent on keeping you safe. But I kept thinking: if I were in this guy's shoes, I wouldn't take you to a hair salon. I'd take you on a hike. I'd take you to the woods and let you explore. You like that sorta stuff, right?' Sans grinned.

Frisk nodded, and tried to still her heart.

'And when that guy forced you,' Sans suddenly quieted. He shook his head. 'I shouldn'ta done that. I shoulda just yelled at him, he coulda gotten hurt or somethin'.'

Frisk stared at her feet. 'It was kind of funny.'

Sans looked at her in surprise.

'I mean, I didn't want him to get hurt, and at the time I wasn't too happy but looking back he sort of looked like a doll or something,' Frisk admitted.

Sans smiled and chuckled. 'Yeah, I thought so too.' He paused before adding: 'He's a pretty guy, though, why'd you reject him?'

'You answer my questions and then I'll answer yours,' Frisk said decidedly.

'No fair. You asked three things.' Sans eyed Frisk warily.

Frisk pursed her lips and stuck out her chin.

Sans laughed and put a hand in his hair. 'You haven't changed since you were twelve.'

'I have, too,' Frisk said defiantly. 'And you're avoiding my last question.' Frisk tilted her head to one side. 'Why did you decide to become human?'

Sans's brow raised again and his cheeks darkened. 'Uh,' he said, and scratched his head. 'Well, you were eating all this food when you first arrived and it looked really good, but it's hard to taste without a tongue.' Sans grinned.

Frisk frowned. 'Before…' Frisk took a deep breath to steady herself. 'When I told you how I felt, you said that if we did anything we'd have to wait five years.' Frisk looked at Sans sternly. 'That's how long you were asleep. Exactly five years.' Her brow furrowed and she did her best to remain strong. 'You asked me not to lie. So...I'm asking you to give me the same courtesy.' Frisk's voice softened and she added: 'Even if the lie is masked as a joke, it's still a lie.'

Sans stared blankly before letting out a laugh. He put his hands on Frisk's shoulders and bowed his head, still laughing, until he was leaning into the nook between her neck and her shoulder. His laughter faded and he went still, holding tightly to Frisk's shoulders.

'Sans?' she asked.

'I did it for you,' he said in hardly a whisper.

Frisk couldn't reply for a moment. 'Me?' she said.

'Yeah. You.'

Frisk put her hands on Sans's back. 'Why?'

'Because…' Sans shook his head. 'Because I loved you. And I was so much older, and you were just a kid. It was wrong, and I knew it, and I thought if I became human maybe I could be around your age. Then when Alphys said it might not work that way, I figured I'd just wait, then. Then Alphys said that my body would take whatever shape I thought I should have, and I kept trying to imagine myself as this buff guy, but that's not how I see myself, it never has been. And that guy, Oliver, _he's_ a well formed guy. Alphys kept talking about how my thoughts would affect the magic and how the magic would make the body, and I was thinking: five years, just five years.' Sans quieted. His grip loosened and he lifted his head. He cleared his throat and shoved his hands into the pockets of his hoodie. 'So...you should just reject me already. I already messed up my body and that's not your fault. None of what I did is your fault. It was all me.'

Frisk stared at him and after several moments of shock, she couldn't help but grin. 'You bone-head.' She leaned her head forward and onto his chest. 'I love you.'

She heard Sans's heart quicken.

'Frisk.' He gently pushed her away. 'Frisk,' he said again.

'I...I didn't stop,' she told him. 'I even tried. I dated people.' Frisk shook her head. 'It was _horrible._ They weren't funny, they weren't smart, they certainly didn't know how to bend the laws of time and space.'

Sans laughed despite himself.

'And no matter what I did, no matter what I tried, I kept missing you and wanting you to come back,' Frisk went on. Tears stung her eyes. 'Toriel wanted me to date Oliver, and when everyone said you might not wake up, I almost did just to see if I _could_ move on, and he may love me, but he's rude and impatient and stupid, Sans.' Frisk was breathless. 'I knew he was going to confess and I really didn't want him to, because I like being Oliver's friend.'

'Frisk,' Sans said.

'And I figured I could let him down gently, I didn't think that he'd go ballistic on me, or that he'd say I _owe_ him, can you believe that?' Frisk was babbling. 'But then the whole day I felt like someone was watching me and I saw Papyrus's cape, and then the blue fire when Oliver was thrown into the lake and I _knew_ it was you, and I _knew_ you became human for me, I just didn't know how to tell you that I haven't stopped thinking about you or loving you and it didn't matter how old I was-'

Sans put his fingers on Frisk's lips. He hesitated before asking: 'May I?'

Frisk had no idea what he was asking, but she nodded.

Sans beant forward and pulled his fingers away. He pressed his lips against hers. Frisk closed her eyes as he put his hands in her hair and as he caressed her lips with his. His magic reacted and blue fire surrounded the two for a moment. A few blissful moments went by and Frisk was locked in the feeling of his lips against hers. He stopped, panting and holding Frisk.

'I've wanted to do that for so long,' he told her.

Frisk put her hand on Sans's neck. He shivered and the blue magic surrounding the two flickered and faded.

'What do we do now?' Frisk asked.

Sans smiled. 'Let's go on a date,' he told her.

Frisk nodded, fighting the urge to kiss him again.

Sans pulled away. 'You gotta sleep tonight, though,' he told her firmly.

 _Can I sleep with you?_ Frisk blushed and nodded.

'Two days from now,' Sans said. 'You and me are going to go on a date.' Sans grinned. 'Think you can wait that long?'

 _Two days?_ Frisk returned the grin. 'I've already waited five years.'

Sans led Frisk to her room and kissed her forehead. ''Night, Frisk.'

Frisk nodded. 'I-I love you.'

Sans smiled. 'I love you, too.'


	7. Chapter 7: Home sweet home

undertale: the human equation

chapter 7: Home, sweet home

Sans:

If happiness had a tune, Sans felt as though he were walking to it. Every step he took was careless and worry-free. He hadn't slept the night before, and despite being tired he was wandering around town with his head held high. He was a free man, drunk on love and his own happiness.

She loves me, he thought. The thought was like a ray of sunshine cutting through clouds. He found himself giggling as he walked through town like an idiot, but he didn't care.

Sans was on a mission.

He wanted to do something for her, something that would make her happy. The two had been playing cat-and-mouse for far too long, and Sans wanted to make up for it. He went to a nearby flower shop and found the perfect sunflower. He purchased it without another thought and quickly hurried home.

As he made his way back to Frisk's house, he caught sight of someone else making their way. Sans froze when he saw Oliver.

Oliver didn't look any worse for wear. He was standing in the garden with Asgore, shirtless, with a hoe in hand. He wiped sweat from his brow, and Sans realized he was helping with the garden. Sans felt planted in place, knowing that if he got any closer, Oliver would notice him. Sans looked down at his protruding stomach and cringed. Why was Frisk in love with him? He didn't understand it, but he wouldn't question it, either. He could still remember the heat of her lips and the feel of her hair in his hands.

 _I didn't force her,_ he thought, giving Oliver a wicket stare. _She wanted to kiss me._ With that thought, Sans stepped forward, clutching the sunflower in the flower pot. Oliver glared and for a glorious moment, Sans felt as though he had won some sort of battle. Then Frisk popped her head above the hedges and caught sight of Sans.

'Sans,' she said with a wide smile.

'Eh, hey,' Sans said. He felt his face getting hot, and couldn't stop it.

'What's this?' Frisk asked, stepping lightly over her flowers.

Sans chuckled and said: 'I got it for you.'

'A sunflower?' Frisk asked. 'Wow it's so tall already!'

'Yeah,' Sans said. 'I thought you'd like it.'

'Do you want to help me plant it?' Frisk asked.

'Sure,' Sans said. He grinned at Oliver as Frisk led him to an empty spot in the garden.

Frisk began digging and Sans crouched on the ground with her.

'Hopefully the seeds will fall,' Frisk told him. 'Then we can have lots of sunflowers.' She looked up and smiled widely. She took the flower pot and carefully removed it, roots and all.

Sans glanced at Oliver, who was speaking and laughing with Asgore. He turned back to Frisk and helped her bury the roots. As she worked her hands through the soil, he put his hands on hers.

Frisk looked up and giggled like a kid. Before Sans could say anything his stomach let out a loud grumble. He blushed and looked down.

'Are you hungry?' Frisk asked.

Oliver snickered nearby.

'Um, it's fine,' Sans started.

Frisk studied him for a moment and stood. 'Well, I'm hungry. I'm going to get some food.' Frisk marched away with her head held high.

Sans's stomach grumbled again and he cursed his human body.

They stepped into her kitchen, which was surprisngly empty. Frisk grabbed eggs, bacon and sausage. _Breakfast_ , Sans thought. _She's so great_.

'Papyrus was looking for you this morning,' Frisk said lightly.

Sans noticed for the first time that Frisk was in shorts. His gaze traveled down her back, and to her backside.

'He said he wanted you to join him during the lightless ceremony a couple of days from now,' Frisk said.

Sans was having troubles staring at anything other then Frisk. He forced himself to look at his hands, as desire coursed through him, making his loins react in ways he didn't know were possible. 'What's the lightless ceremony?' Sans asked.

'Well, a week ago everyone was celebrating summer,' Frisk said. She had finished cooking quickly and was scooping everything onto a big plate. Frisk turned and put the plate on the table, leaning forward and exposing her cleavage. 'So, tonight through the next three days, the whole town will turn off the lights and everyone will come to look at the sky.'

Sans forced himself to look at Frisk's face. 'Sounds good,' he said. His stomach grumbled and Frisk laughed.

'Well, since you're not hungry, I guess I'll eat all this by myself.' She said and grabbed a forkfull of egg.

'Good, I rarely see you eat at all,' Sans replied dryly. He touched his stomach cursing himself for being hungry and cursing his stomach for being large and unsightly. 'At this rate, you're going to shrivel up and dissapear.'

'I'm not the one whose stomach is growling like an unfed bear,' Frisk said with a shrug.

'You're also not the one of us that needs to lose weight,' Sans said.

Frisk rose an eyebrow. 'Didn't Alphys say that this body is how you see yourself?'

Sans shrugged and took a bite of the eggs. 'I'm not uncomfortable,' he admitted. _Although I wish I looked a little better for you._

'I think you look good,' Frisk told him.

Sans couldn't help but smile. He took another bite of the food. 'Well, that makes one of us.'

'Sans!' Papyrus stepped into the kitchen. 'There you are. I was working on a puzzle for the humans in the village. The smaller ones seem to like them quite a bit!'

Sans wanted to tell his brother to wait, but Frisk smiled and stood. 'I'm supposed to go to the town hall today,' she said kindly.

'Right,' Sans said _. I want to spend the day with you, though,_ he thought.

Asgore called and Frisk hurried outside. Sans watched and put his hand on his chest. When did all these feelings appear? Sans had expected rejection, and instead found acceptance. He touched his stomach and tried to think strait. It wasn't just acceptance, he felt inadequate in comparison to Oliver. Frisk was about to walk towards the town hall with Oliver at her side and Sans couldn't do anything about it. Despite his actions, he had been her friend, and he was her body guard. Sans put a hand on his chest.

'Sans, you look terrible,' Papyrus said. 'Working on puzzles will be fun! And Alphys says that Mettaton makes wonderful burgers.'

 _Burgers_. Sans's mouth watered. He grabbed a plate and filled it with eggs. 'Alright, Paps. For the burgers.'

'For the puzzles, Sans, Toriel is taking the children on a field trip to the underground,' Papyrus said.

Frisk:

Since the date with Oliver, Asgore had taken to walking with Frisk into town. Frisk knew that her parents were fond of Oliver, but Asgore has a sixth sense when it came to Frisk's emotions.

Frisk didn't make a habit of showing her true feelings to everyone. That was something she and Asgore had in common. She was an expressive creature, but she was expressive on purpose. Anyone who glanced at Frisk assumed that they understood what she was feeling and why. That couldn't be farther fromt he truth. She kept her innermost thoughts and feelings behind high walls carefully crafted to keep anyone from getting too close.

Oliver had assumed Frisk was a soft and easy girl that he could take advantage of. The thought stung Frisk, and she didn't understand why he was still hanging around. Oliver had thrown on a shirt before they left and walked proudly, but Frisk knew he was angry. Who wouldn't be? Not only had she rejected him, but he had been tossed into a lake.

 _Sans_. Frisk's heart burned. Sans had protected her. She almost snickered; she could imagine Sans lifting his hand and flinging Oliver around. Sans looked soft, but he was a dangerous foe to have. Sans hid his true colors from everyone, too.

The meeting in town hall was mostly about public works projects and how to incorporate projects for monsters. Frisk was delighted at the prospect of more activities for her friends. At the end of it all, she and Asgore left for Frisk's house and Oliver stayed in town.

As they walked alongside the lake, Asgore sighed and looked around himself.

'I wish summer didn't have to end,' Frisk said suddenly.

Asgore looked at her and smiled. 'All things end at some point.'

Frisk's heart faltered and she looked down.

'The old things go away to make room for the new,' he said as they walked. 'I think the only exceptions to that rule are things like hate and love.'

Frisk nodded, but the notion of having to lose things made her sad.

'Oliver told me he confessed,' Asgore said lightly.

Frisk had been dreading conversing about Oliver to her parents. 'Yes,' she replied. 'He did.'

'He was not very happy with your response,' Asgore went on. 'Niether was Toriel.'

Frisk chuckled. 'She really liked him.'

'She likes the idea of him,' Asgore replied firmly. 'He knows how to please people.' Asgore eyed Frisk kindly. 'Why did you not accept his love?'

'I don't feel that way about him,' Frisk said. 'I never have.'

Asgore nodded. 'Because of Sans.'

Frisk's heart skipped a beat. 'How...how did you know that?'

Asgore laughed and patted Frisk's back. 'You give me too little credit.' He calmed and stopped walking to watch the sun set on the lake. 'Did Sans...try anything with you?'

Try anything? 'What do you mean?' Frisk asked, but she had a sinking feeling she knew.

'When you were younger,' Asgore replied. 'When you first discovered your feelings, was it because of your heart or because he had done something?' Asgore sounded uncomfortable but he was determined. 'I mean sexually, Frisk.'

Frisk blushed and shook her head. 'N-no, he never did anything like that,' Frisk said quickly.

Asgore simply smiled and turned back to the lake. 'Oh, I know. I just wanted to hear it from the horse's mouth.'

Frisk felt like she had just run a marathon. She, too, turned to the lake.

'May I ask something?' Asgore said.

'Sure,' Frisk replied, but she almost said: no. She was half afraid he'd ask another question about sex with Sans and Frisk wasn't sure how to handle that.

'Before Sans decided to become human, he came to me,' he said. 'He asked how I knew that I loved Toriel.'

Frisk tilted her head to one side.

'I wasn't really sure how to answer at the time,' Asgore went on. 'I still don't quite know if I have the words. Even after everything that's happened, even though she's done her best to distance herself from me. I still want her, and I still love her.' Asgore quieted and his eyes glazed over. 'Sans told me about your confession back then.'

Frisk was shocked. Toriel had been angry, but Asgore didn't seem the same.

'You were so young to make such a bold statment,' Asgore said. 'Even now, you hold true to those feelings.' Asgore turned to Frisk.

'Oliver was trying to get you to confess to Toriel,' Frisk said. 'Aren't you holding true to her, too?'

Asgore smiled sadly but turned his head. 'I don't know if she'll still have me,' he admitted.

'Haven't you told her?' Frisk asked, clenching her fists.

Asgore chuckled and shook his head.

'Father,' Frisk said. 'You can't keep chickening out.'

Asgore rose his brow in surprise. 'I'm not,' he said.

'Then, you've told her you're still in love with her, right?' Frisk asked.

Asgore blushed and shook his head.

'Well, then you know what you have to do,' Frisk said.

Asgore sighed and shook his head. 'She'll deny me,' he said hopelessly.

Frisk couldn't help but laugh. 'How do you know?' She turned to walk towards the house.

Asgore followed, and put his arm around Frisk's shoulders. 'Do you know if Sans loves you?' Asgore smiled. 'It almost seems like it should be obvious, but to those involved, it's like walking through a fog.'

 _Maybe she'll be at the house_ , Frisk thought. _Maybe I should text mom, and have her come over tonight_.

A strange sight greeted Frisk and Asgore. Just beyond the crest of the hill, smoke billowed into the sky. Frisk saw it first and froze. Her heart fell into her stomach and as Asgore noticed, the two exchanged a worried look. Then they both broke out into a run towards Frisk's house.


	8. Chapter 8: Too much

undertale: the human equation

chapter 8: Too much

Sans had eaten too many burgers. While Papyrus thought up a brilliantly complicated puzzle and got to work setting it up, Sans put an etch n' sketch on a mound of dirt and went to the Mettaton Hotel. Not only had he eaten as much of breakfast as he could, but he had also found the kid selling burgers and began binging. Sans had no shame and sat on one of the balconies of the hotel, so stuffed he could hardly moved.

He closed his eyes and leaned his head against the wall behind him. _I could just rest my eyes,_ he thought. _No one would notice_. He was so warm and full that he couldn't help but doze off. After a while, he heard some kind of commotion downstairs, but didn't pay it any mind. He continued to doze, half asleep, until he heard Frisk's name.

'...it's not the house, b-but Frisk sounded awfully upset!' That was Alphys's voice. Sans's eyes snapped open.

He teleported downstairs just behind Alphys. For a moment, Sans's stomach turned and he cursed himself for eating too much.

'Alphys,' he said.

Alphys jumped and turned around. Mettaton was walking beside her, carring a sleeping Papyrus under his arm.

'Oh, Sansy, the most audacious thing has happened,' Mettaton cried.

'We need to get to Frisk's house!' Alphys said. 'I already called Toriel and Undyne. Hurry up, all of you!' Alphys turned, walking faster then Sans had seen her move in a long time.

'What's going on?' Sans asked.

'S-someone set fire to Frisk's garden!'

Sans's heart fell into his stomach.

'It's an absolute tragedy,' Mettaton cried. 'My fans must see it.'

'Paps,' Sans started.

'Oh, he fell asleep while we were having a dance off,' Mettaton said with a laugh.

'Right. I'm gonna go ahead,' Sans said.

'Be careful! We don't know how this started!' Alphys called.

Sans couldn't teleport very far, especially with how much food he had eaten. Every muscle and bone cried out for him to relax and go find a place to nap. Frisk might be in danger, though, and Sans had no faith that the pretty-boy-bodyguard was doing much to protect her. He kept moving himself, trying to teleport longer distances each time. When Sans reached the stairwell leading to the surface, he was too exausted to keep teleporting. He glared at the stairs and looked up to the sky above. He might need his magic to help Frisk anyway, and if he overwhelmed himself he'd pass out regardless of what happened.

Sans hurried up the stairs. When he reached the top, he was met with the sight of the burning garden.

The fire had grown so large that the townsfolk, as well as monsters from below had emerged to help put it out. A large firetruck had parked on the dirtroad out front. People had been dumping buckets of water onto the ground. Sans looked around himself wildly and caught sight of Asgore holding tight to a large tube of water. The firemen did something and water shot out of the hose. Asgore held it steady.

'Frisk!' Sans yelled over the people shouting and running around. Dammit, where is she? 'Frisk!' he yelled again and ran towards the house. He felt sick but he willed himself to keep moving.

The house groaned as soon as he stepped inside of the kitchen. Sans froze; the sound wasn't normal. 'Frisk?' he called. 'Kid?' The house groaned and when Sans took another step the floor creaked and shifted. Sans froze, staring at the floor, until he heard footsteps and something crash in the other room.

Frisk ran out of Papyrus's bedroom with an arm full of items. 'Sans!' she cried.

'Frisk, what're you doing? What's going on?' Sans asked, full of worry.

'I don't know,' she said, 'but I think the house might collapse!'

'Shit, yeah, I can hear it,' Sans said. He stepped forward, ignoring the creaks beneath his feet and grabbed Frisk's arm. The house let out another groan and Sans put his arms around Frisk. A beam let out an awful creak as it splintered, and the wall of the kitchen went with it. Sans lifted a hand and used his magic to sheild the two, but Frisk shrieked. Sans panicked, and the floor creaked and split open beneath their feet.

Sans let out a cry as the floor gave way and the two tumbled into the darkness below. With a rush of adreneline, Sans clung to Frisk with one arm and used his magic to slow their descent. He had used too much, though, and it was making him weak. Sans did his best to try and see, but it wasn't easy.

The two hit the ground and Sans mercifully released his magic. Sans had been able to save them, but they still hit the ground hard. Frisk had landed on her back, and Sans had heard her head hit the ground. He had landed ontop of her, feeling the full length of her body pressed against his. For a brief moment he relished in it, before lifting himself. The glow of his magic illuminated very little in their surrounding, but Sans could see no blood. Frisk looked around dizzily, her eyes getting droopy.

'Hey, hey, hey,' Sans said, touching the side of her face and gently tapping her cheek. 'Stay awake. Are you hurt?' Sans's heart beat wildly in his chest. He could smell smoke coming from somewhere, but he didn't care.

'I-I hit my head,' Frisk said. She looked around with wide eyes. 'I'm seeing stars.' Her eyes went droopy.

'C'mon, kid, sit up with me,' Sans said, trying to pull her up. Frisk let him move her easily but she rubbed her head.

'It hurts,' she said. She suddenly looked around. 'Pap's things-the house!'

'S'all right, we'll figure that out later,' Sans said.

Frisk let out an exasparated sigh. 'Sans,' she said. Her lip trembled. 'I don't know what happened.' Tears filled her eyes and she put her hand in her hair. 'What am I going to do? Where am I going to live?'

'Shh, shh, we'll figure out all that later,' Sans said kindly. 'I think you may have gotten a concussion when you fell.'

Frisk's lip trembled again, and despite his worry Sans's heart melted. 'The sunflower was gone,' she wailed, wiping tears from her eyes. 'I wanted to save it,' she said.

Sans couldn't help but chuckle and pulled her close. 'It's fine, Frisk, I just want you to be okay.' Sans paused, still unable to see much even with his glowing magic radiating blue light from his eye. 'But we need to figure out where we are.'

'I'm so tired,' Frisk said.

'Yeah, but you're gonna stay awake,' Sans said. He stood and tried to peer through the darkness. 'It's no wonder your house fell if there's a bunch of caves under here,' he said, scratching his head. He tried to use his magic to light up the surrounding area, but as he did, his bones began to ache. He could only use his magic close to his body. Sans sighed and lowered his arm.

'Sans?' Frisk asked.

'I'd teleport us or light things up, but...I used a lot of my magic,' he told her. Even though most of it I used to just teleport here, he thought. 'Hey, I'm gonna look around,' he started.

Frisk nodded but Sans worried. If he left, she'd have to sit alone, in darkness.

Sans suddenly bent down. 'C'mere,' he said.

'What are you,' Frisk started.

'Well, I don't want you to sit in the dark and fall asleep, and I'm half worried that some piece of your house will fall down here,' Sans said truthfully. 'So, I'm gonna carry you.'

'I can walk,' Frisk protested.

'Alright.' Sans stood and took a step back.

Frisk stood unsteadily and stepped forward. Sans almost laughed when she tripped over nothing. He caught her easily. 'I mean, do you want to wait here, all alone in the dark?'

Frisk shook her head.

'Then get on my back,' Sans said and knelt down again.

Frisk put her arms around his shoulders and wrapped her legs around his waist. As Sans stood, it was the first time that he was thankful his human body was thicker than most others. Frisk could hang on to him easily, but her little body pressed against his distracted Sans for a moment. He stood still, trying to calm the urges coursing through him.

'Sans?' Frisk asked. Her voice was right next to Sans's ear, and once again he found himself fighting his own urges.

'D-did you hear about the guy that got hit in the head with a can of pop?' he asked.

'No,' Frisk said. Once again her voice and breath against his ear made Sans's loins harden.

'He was lucky it was a soft drink,' Sans said and stepped forward.

Frisk laughed and put her head on Sans's shoulder.

'I'd tell you a chemistry joke, but I'm not sure if I'd get a reaction,' Sans went on, as he walked down the cave.

'Hopefully it isn't too watered down,' Frisk added.

Sans chuckled, still trying to calm down.

'Although if our chemistry is just right, things could really come to a boiling point,' Frisk went on.

Sans broke out into a laugh and stopped walking for a moment. You have no idea, he thought.

Sans walked for a little while, and he began to get sleepy himself. It had been about an hour, but because Sans had used so much of his magic, and because his loins ached from forcefully willing away his desires, he was left exhausted.

'Frisk,' he said at one point. 'I gotta sit.'

'Sans.' Her voice at his ear only started up Sans's problems all over again. 'There's something up ahead.'

Sans blinked and squinted. Frisk was right. Sans could even feel a draft.

'Okay,' Sans said. 'Just a little farther.'

Sans willed himself to keep walking, even though his legs and muscles ached. As he continued, he noticed the sky outside. When they got to the opening, Sans saw that they were at an opening overlooking the lake. He gingerly put Frisk down and stared at the sky and the lake.

The moon hung like a silver coin in the sky. Its light wavered on the surface of the lake. The stars glimmered like diamonds in the sky.

'It's so pretty,' Sans said.

Frisk leaned against the wall of the cave. 'My house was sitting on a cave this whole time,' she said with a laugh.

Sans sat beside Frisk and leaned with her. 'I'm beat,' he said.

Frisk gave him an odd look. 'I'm sorry,' she said quickly.

'No, it's fine,' Sans said. 'I just overworked myself.' Sans's stomach grumbled and Frisk simply stared for a long moment. Sans laughed. 'You know,' he said, 'I overate this mornin'. I don't understand why this body needs so much food.'

Frisk leaned against Sans's shoulder. He nearly jumped; he felt sensitive to everything and it was driving him nuts.

'I can make you something other than breakfast,' Frisk offered. 'Like...french toast or pancakes.'

'Frisk...that's all breakfast food,' Sans asked.

Frisk didn't answer for a moment. 'I burn everything else,' she admitted and put her hands to her face. 'I can only make stuff in the morning.'

Sans laughed and ran his hand through his hair. 'I'm sorry, kid, if I were a little stronger I could get us out of here.'

'It's alright,' Frisk told him. 'But...I'm really tired.'

Shit, it's cold, too, Sans thought. He took off his hoodie and slid it around Frisk's shoulders. At least that helps me calm down, though. 'C'mere,' Sans said, holding his arms open.

He gingerly laid Frisk down on his lap and stroked her head. 'Won't you be cold?' she asked.

'I don't mind,' Sans said. 'It's actually warranted for me. But...I'm sorry, kid. It looks like we're sleepin' here tonight.'


	9. Chapter 9: Home bones

Undertale: The Human Equatino

Chapter 9: Where Frisk Will Live

Frisk:

Sunlight touched Frisk's eyelids. Her back felt stiff and a dull ache touched the back of her head. Frisk opened her eyes and listened to the sound of waves crashing against the cliffs. Her brow furrowed and she tried to recall where she was. She sat up slowly, putting her hand on her head. Her head didn't hurt badly, but she was having troubles thinking.

'Hey, kid.'

Sans was sitting propped against the wall. He looked tired. His white hair was disheveled and he had bags under his blue eyes. He offered a slight smile.

'Sans,' Frisk said. She looked back out at the lake. 'Where are we?'

Sans tilted his head to one side curiously. 'You don't remember?'

'I remember…' Frisk closed her eyes. 'Dad and I saw smoke coming from home...and we ran towards it, and the garden was on fire.'

Sans eyed Frisk, but try as she might, she couldn't remember anything else.

'I...I think I ran into the house, but…' Frisk shook her head. 'It's all foggy beyond that.'

'The house collapsed,' Sans said. 'We fell down here.' Sans scratched his stomach. 'I kinda overdid it with my magic. It's my fault we couldn't get outta here, but I should be able to do somethin' now.'

Frisk was happy that they'd be able to get out of the caves but she couldn't hide her worry. 'Where am I going to live?' she asked, but wasn't expecting an answer.

Sans got a strange look in his eyes, but shook his head and patted Frisk's shoulder. 'Don't worry about that yet,' he said. 'Let's get outta here.'

Sans used his magic to make a crude stairwell along the side of the cliff. Together, the two carefully picked their way to the little sandy beach near the town. Frisk stepped lightly from the cliff to the beach and Sans followed. His stomach was grumbling and it almost made Frisk laugh. She had already decided to make him french toast as they were walking towards the town, when she felt raindrops on her head.

Sans stepped beside Frisk, and waived his hand. Blue magic sheilded the two as rain began to fall steadily. As they trudged up the grassy hill, Sans looked around curiously.

'Isn't this where Oliver…?' Sans quieted and rubbed his neck.

'Oh,' Frisk said suddenly. 'I hadn't even noticed.' She didn't make a habit of thinking about Oliver, and she hadn't been happy at all that night. Sans was right, though. They were walking along the grassy feild where Oliver had forced Frisk into a kiss.

Sans took Frisk's arm. She blushed and turned his way.

'I hope this is alright,' Sans said carefully. 'I dun' want to do nothin' that would make you uncomfortable.'

Frisk smiled. _Is he thinking about that day?_

Frisk took his hand in hers and offered another smile. 'How about this?' she asked bravely.

Sans looked at their hands and laughed. 'This is good.' He added: 'You look pretty cute wearing my hoodie, too.'

They walked quietly and Frisk squeezed his hands. 'It isn't the same, you know.'

'What's not the same?' Sans asked.

'I didn't want to be with Oliver,' Frisk said carefully, as they stepped onto the main street of town. 'I want to be here with you.'

Sans's brow rose.

'And, I know you wouldn't hurt me intentionally,' she went on. 'So…' she turned and grinned. 'You'd have to try really hard to make me uncomfortable, you know?'

Sans nodded and smiled.

Frisk's house sat near the hole leading to the underground in smoldering ruins. Frisk stared, wide-eyed and looked at everything sadly. She had been happy to have her own home, and she liked it when people visited and living with the skeleton brothers. Her heart fell at the sight of everything. She wanted to see her mother; Toriel would probably let her live in the ruins.

'How the hell did the fire start?' Sans asked. 'It isn't easy to set fire to a garden, unless it's dried out.'

'The sunflower,' Frisk started.

Sans shook his head. 'Think nothing of it,' he said. 'There will be plenty of flowers in the future, I promise. I'm just glad you weren't hurt.' Sans scratched his head. 'Aside from the concussion. Sorry.'

 _But what now?_ Frisk thought. She stepped through the muddy, ash filled garden.

'What are you gonna do?' Sans asked curiously.

'I want to see mom and dad,' Frisk said. 'They'll help me think of somewhere to live.'

Sans nodded as though he had expected Frisk's answer. 'I'm gonna come with you,' he said. 'I hope you don't mind.'

'Not at all.' Frisk was actually a little relieved.

Sans took Frisk's hand. 'Then let's go see Tori in the ruins.'

The two descended down the stairs in silence. Although they said little, Frisk was happy to have him at her side. After entering the ruins they found Toriel sitting with Asgore in the living room. They were in a deep conversation; Asgore sat at a table and Toriel was in her chair next to the fire. Once Frisk and Sans stepped into the room, they both paused.

'Frisk!' Toriel cried and stood.

'Hey,' Frisk said.

Toriel ran forward and threw her arms around Frisk. 'Oh, my sweet child, I was so worried!' She pulled away and looked at Frisk carefully. 'Are you alright? Are you hurt?'

'I'm alright,' Frisk said. 'Sans,' she started.

'Sans went after you in the house,' Asgore finished with a sheepish grin.

Toriel lifted her gaze and eyed Sans gently.

Sans shrugged. 'It wasn't a biggie. The house wasn't on fire, I just didn't expect it to collapse.'

Toriel suddenly threw her arms around Sans.

Frisk looked at her father, who simply shrugged.

'Oh, Sans,' Toriel cried. 'Asgore said he saw you run inside of the house, but by the time I got there it had already collapsed.'

'I wouldn't let nothin' bad happen to the kiddo,' Sans admitted and patted Toriel's back. 'I didn't see Paps in the house.'

'He returned to your old home with Undyne and Alphys,' Asgore said.

'Sans,' Toriel said. 'I'm...I'm sorry for being angry with you before, about becoming human. I just, I was worried that perhaps something had happened between you and Frisk, and I didn't want her to get hurt.'

Frisk tried to say something, but Asgore put a fuzzy paw on her arm.

'I know, Tori,' Sans replied. 'I was pretty disgusted with myself, too.'

Toriel laughed.

'Speakin' of the old house in Snowden,' Sans said, 'I was thinkin that Frisk could come and live there.'


	10. Chapter 10: Close call

Undertale: The Human Equation

Chapter 10: A new house

Sans:

Toriel blinked in surprise, and Sans half expected her to say no. When she smiled warmly and said: 'I think that sounds like a good idea.'

It was Sans's turn to stare in surprise.

'Your house?' Frisk asked. 'There's only two bedrooms there,' she started.

'We'll figure out the logistics later,' Sans said. He rose his brow at Toriel. 'You're really okay with it?'

Toriel chuckled and looked from Frisk to Sans. 'Well, you were already living together,' she said. 'And...I've spent enough time being bitter and angry. Frisk is old enough to make her own decisions, now.'

Asgore stepped from Frisk's side and stood beside Toriel. He put his arm around her shoulders.

'But, eh, that's of course if it's what Frisk wants.' Sans put his arms on his hips. 'What d'ya say?'

Frisk couldn't help but smile and nod. 'Yeah,' she said, but she had dozens of questions. Sans could see it in her eyes, but he didn't want to spoil his plan by speaking too soon.

'By the way,' Sans went on. He tilted his head to one side. 'Do we have any idea how this happened?'

At that question, Toriel and Asgore adorned darkened expressions.

'I mean, what with all the moisture in the air preceding this morning's rain and the fact that you and Frisk kept her garden well watered, I can't imagine setting fire to it was an easy task,' Sans said.

'It was set by gasoline,' Asgore said. 'And we aren't sure who did it. We found a few empty canisters of gas hidden in a line of shrubs nearby. Alphys was dusting for fingerprints in her lab.'

 _Then that's where I_ _need to be, too. '_ Our friend Oliver wouldn't have happened to been around, would he?'

Toriel shook her head. 'Despite...not wanting to believe it, I looked into it already. He was at a party with some of his human friends.'

 _I'd bet my body he found a way to do it,_ Sans thought. Before he could voice his thoughts, Asgore spoke up yet again.

'I think you should go see your brother,' the king said. 'He saw you run into the house and became intensly worried.'

Ah, Paps. 'Well, I'm hungry anyhow. And I haven't really been home in five years.' Sans eyed Frisk. 'Shall we?' he asked.

'Yeah,' Frisk said. She couldn't keep the uncertainty from her voice. 'You two,' she started.

'We'll be fine here,' Asgore said kindly. He took Toriel's hand and smiled. 'There are some things I'd like to talk with Toriel about alone.'

Frisk blushed and Sans tugged her away.

While they walked out of the ruins and into the snowy forest, Frisk spoke up almost immediately.

'Do you think they're getting back together?' Frisk asked.

Sans shrugged. 'I hope so,' he admitted. 'Tori's a good woman. Asgore made mistakes. That's all part of life, isn't it?'

'Sans,' Frisk started. A few of the residents of Snowden said hello and Sans waived at them.

Sans's stomach grumbled. 'Jeeze I'm hungry,' he said. He grinned at Frisk. 'After we check out Paps, maybe we can grab a bite to eat.'

Frisk nodded but Sans couldn't help but notice her red cheeks.

'What's on your mind?' he asked.

'Well--your old house only has two rooms,' Frisk started.

The two had just reached the front door of Sans's old house. It suddenly burst open and Papyrus stepped outside.

'Paps,' Sans said.

Papyrus threw his arms around Frisk and tackled her to the ground.

'HUMAN!' he cried through spectral tears. 'I WAS SO WORRIED!'

Undyne appeared in the doorway. 'Paps, I told you she'd be okay.'

'I'M SO SORRY FRISK,' Papyrus cried. 'I COULDN'T SAVE THE HOUSE!'

Sans teleported inside next to Undyne and she jumped. 'Hey, there, bone head,' Undyne said as Frisk and Paps wrestled. Undyne suddenly got a ridiculous grin on her face. 'I hear you have a crush on someone.'

Sans rose his brow. 'Oh? Hear that from Alphys?'

'I have my ways,' Undyne said.

'Definatly Alphys,' Sans replied.

'I've been wanting to punch you for about a week,' Undyne said with a hard frown.

'Only a week?' Sans asked with a wide grin.

Undyne put her hands on her hips. 'Paps was all alone!' she said sternly.

'Yeah, yeah, I made a mess,' Sans said. 'But Paps didn't miss me one bit, right bro?'

'Absolutely!' Papyrus cried standing. Frisk dizzily stood. 'I did not miss him, although I am happy to be back in old house. I wonder if my old race car bed is still there!' Papyrus suddenly ran inside.

'He, uh...he slipped on an ice cube and passed out until just a few minutes ago,' Undyne said. She shrugged.

Sans was glad to be back in his own house. Even though it had been years, it still looked the same. Alphys was eating popcorn on the couch, and Undyne quickly went to sit with her.

'We've just been hanging out and waiting for you two to show up,' Undyne said.

'Yeah, Frisk is gonna live here for a while,' Sans said. 'I'm gonna go check out the room. Frisk, you should come with me.'

'Ah, okay,' Frisk said.

'Wowie, nothing has changed!' Papyrus cried from his room.

Sans trudged into his room and Frisk followed. Once they were inside, he turned on the light and waived his finger. The door shut. Sans's room was a mess, just the way he had left it. Though his swirling vortex of garbaged had blown its self out, everything was the same. He looked from his bed to his dresser, and then to Frisk.

'Sans, how is the going to work?' Frisk asked. Sans had expected the question. 'There's only two rooms.'

'I can sleep on the couch,' Sans said, scratching his head.

Frisk blushed and shook her head. 'No, I'm a guest here, I should be the one on the couch.'

Sans eyed her and grinned. 'I'm not gonna argue with that. My room is a mess and I'm not sure what you'll find in here.' He shrugged and scratched his chin. 'Unless you want to share the room.'

Frisk froze and stared blankly for several seconds.

'I'll take the couch,' Sans said. The two sharing one room was a dangerous idea, and he knew it. Just the night before he had been barely able to control his primal urges, and he had no idea what would happen if they shared a living space. The thought of her undressing sent shivers through Sans. 'I'm just messin' around,' he added. He turned to leave.

'Wait,' Frisk said.

Sans was tense, himself. He knew he was asking a big thing, which was why he hadn't told Toriel or Asgore about his little plan.

'What if we did?' Frisk asked.

'Then, I guess this'd be your room and mine,' Sans said with a shrug.

Frisk blushed and looked at her feet. 'I'd be okay with that.'

Sans couldn't help but grin. 'Just...one thing I wanted to talk to 'ya about.'

'What is it?' Frisk asked.

 _I'm a guy, you know. What'll you do if you figure out all the dirty things on my mind_? Sans didn't have the guts to admit what he was thinking. 'Where are we gonna get you some clothes?' Sans asked. 'And...can we get some food?'

Frisk laughed, which was like music to Sans's ears. 'I thought you were going to admit some deep dark secret to me.'

 _Well, I chickened out_ , Sans thought. Sans shrugged. 'It's just--all your stuff is underneath your house and I figure we should find something.' Sans's stomach grumbled. 'And apparantly that isn't going to stop 'till I eat.'

'Grillby's,' Frisk said. 'I can run and grab something for you.'

'Yeah,' Sans said, and before he could argue, Frisk was out the door. Sans stood stock still and put his hand in his hair. _This isn't a bad thing_ , he thought. _We were already living together before_. It was different, though, and Sans knew it. He looked at all of his old things _. The last time she was in here, she was tellin'_ _me she loved me, and I was rejecting her. Now she's going to live here. She's going to sleep in this bed._ Sans rubbed his jaw _. I should probably clean up a little._

Frisk:

Frisk hurried out the door and stepped into the snowy ground of Snowden. She walked briskly and put a hand in her hair.

 _We're going to live in the same room_? She felt like she might fall over if she didn't concentrate. Though she couldn't remember what had happened the night prior, Frisk was keenly aware that she had been thinking sexual things about Sans for quite a while. What if something happened? Frisk stopped at the front door of Grillby's and put her face in her hands. She couldn't tell whether she should be giddy or frightened. She hadn't ever had sex before. She didn't know what she should expect under normal circumstances, let alone sex with a monster-turned-human.

Frisk was barely able to stammer out an order to the silent Grillby at the counter of the bar. When she got a burger and fries, she quickly headed back to the house. Once inside, she found that Alphys and Undyne had left, leaving a note taped to the tv.

'Paps challenged Undyne to duel, heading to Waterfall to PROVE THAT I AM THE BEST'

Frisk let out a sigh.

'Everyone cleared out, huh?' Sans asked.

'Papyrus always does this and he ends up either tripping and passing out from falling over or from exaustion,' Frisk said. 'I got you some food,' she said.

'Cool. I'll be in the room,' he said, and stepped away.

Frisk hesitated, staring at the open doorway. _It's alright_ , she thought. _We were living together before_. She walked up the stairs and stepped into the room.

Sans had cleaned a lot in a short period of time. The garbage had been piled into a trash can and everything had been dusted. Frisk even smelled a hint of lemon, as though he had sprayed around before she entered. Sans patted the bed next to him where he sat.

Frisk sat down gingerly and as she handed Sans his food she realized just how tired she was.

'It's been a long time since I've been in here,' she remarked, and leaned back on her hands.

Sans eyed her warily. 'Last time you were, I thought I was gonna have to hurt you for good.'

Frisk looked at her feet. _I never thought I'd be in this position, she thought._

'Jeeze, it's already eleven,' Sans said putting his hand in his hair.

'Yeah,' Frisk said. 'I didn't even realize how long everything took today, and you were right. I have no idea how that fire started.'

'We can just sleep on it,' Sans said. He glanced at Frisk. 'You can borrow one of my shirts to sleep in.' Sans stood and stepped through his doorway.

'Ah. Right.' Frisk stood but before she could do anything, Sans's blue magic opened his closet and one of his shirts landed on the floor.

'I'm not lookin,' Sans said. He stood with his back turned.

Frisk gingerly removed Sans's hoodie and her own striped shirt before putting on Sans's old white one. She blushed and paused, sniffing the thing. It smelled like Sans. 'I'm done,' she said.

Sans lumbered back into the room and closed the door. He scratched his head and said: 'I'm just gonna lay down.'

Frisk nodded, but she felt tense. Sans plopped into his bed and she sat down gently. When she laid down she turned her back on him and did her best to become comfortable. Sans moved his hand and the light switch turned, blanketing the two in darkness.

After a few moments of tense silence, Sans said: 'Hey, kid.'

'Yeah?' Frisk said.

'There's somethin' I've wanted to do do for a while,' he said. 'May I?'

Frisk felt the blood rush to her face and she gripped the sheets. 'Um,' she started.

Sans slid his arm around Frisk, and pressed himself against her. Frisk closed her eyes as Sans put his hand on her stomach. She put her hand on his, relishing in the feel of his body against hers. Frisk's heart beat against her sternum, as though it might break free at any moment.

'Yer heart is pounding,' he remarked.

'So is yours,' Frisk said, almost defensively. She could feel Sans's heartbeat, too.

Sans laughed. 'Dun' worry,' he said, 'I won't do anything that we couldn't talk to Tori about.'

Frisk felt like she might explode if he kept speaking. 'L-like what?' she dared ask. She knew that Sans was implying something sexual, but she wanted him to say it.

'I thought I was being obvious,' Sans said.

Frisk gulped and tried to calm down.

'Why?' Sans asked. 'What do you think I mean?'

 _He's turned this around on me!_ Frisk gripped his hand. She closed her eyes and took a deep breath. 'I've never slept with someone like this before,' Frisk told him.

Sans tensed, and for a moment Frisk was proud of herself. She had spoken truthfully, while making an inuendo at the same time. As she was basking in her moment of triumph, she felt something poke her back. Before she could ask anything, Sans laughed.

'You're getting more ridiculous the older you get,' he said and squeezed her.

'How are we supposed to sleep like this?' Frisk demanded.

'It's easy just close your eyes,' Sans said. Something poked Frisk's back again, but she didn't dare ask what it was.

'My eyes are closed,' she told him.

'Then just calm down,' Sans said.

Both were quiet for a moment.

'We don't need to do anything but sleep,' Sans said. 'I just...wanted to be close to you.'

Frisk did calm down, then, but she knew that Sans was having just as many horrible thoughts as she was, because he was still poking her in the back. It didn't take a genious to realize that he had an erection, but she couldn't just blatantly ask him about it. As his breathing slowed, the poking subsided and he eventually fell asleep. Frisk fell asleep quickly after, a little disappointed that Sans hadn't done anything. Part of her wondered what it would be like to have sex with him, even though it embarassed her to admit to herself.


	11. Chapter 11: Tension

undertale: the human equation

chapter 11:

Sans:

Who did it? Sans wanted to know, and he would do his best to find out. Someone had burned Frisk's garden, and because of it, she could have gotten hurt. She _did_ get hurt. Even though Sans had saved Frisk from her crumbling house, she had gotten a concussion. Someone had tried to hurt Frisk and it made Sans unbelievably angry.

When Sans woke, though, with Frisk in his arms. For a few shining moments, everything else faded away as he stared at her sleeping face. She looked so peaceful, and Sans almost laughed when she let out a snore. _She snores,_ he thought with a giggle. _I'm never gonna sleep again._ That wasn't true, but he was happy nonetheless.

Then his gaze drifted to her chest, where his shirt had fallen astray. Her cleavage sat right in the open and Sans reacted without thinking. His loins got hard and he suddenly sqeezed his eyes shut, willing himself to calm down. He was getting used to moments like that, where Frisk would unknowingly do something that drove him wild. _Jesus, she's asleep,_ he thought. _You're being ridiculous._ Sans snickered again, but this time it was because he made a pun: _Ri-dick-ulous._

Frisk groggily opened her eyes and Sans went into a panic. He was still hard as a rock and she had no idea. Once Frisk saw him she broke out into a smile. 'Oh, hello,' she said sweetly. 'I forgot where I was for a moment.'

Sans studied her sleepy little face for a moment. 'I thought I might still be dreamin,' he said gently.

Frisk blushed and laughed. It sent shivers through Sans, and he tried to fight all his primitive urges.

'So, how was it?' Sans asked.

'How was what?' Frisk asked, rubbing her eyes. She hadn't realized that she was showing off her cleavage, but Sans wasn't going to argue with the sight.

'Your first time sleepin' with a guy,' Sans said with a grin.

Frisk blushed and suddenly sat up. 'We didn't,' she started.

'I meant literally,' Sans said with a chuckle.

'Oh,' Frisk said. 'Right.' She had moved her shirt to hide herself, which was a shame considering how pretty she was to Sans.

'You didn't answer my question though,' Sans said.

Frisk merely smiled. 'I slept like the dead.'

Sans's stomach grumbled. He cursed himself and his body, as per the usual whenever he got hungry.

'I can go make food,' Frisk said immediately and stood up. Before she could scurry away, Sans used his magic to tug on her wrist. She gave him a questioning look.

'Let's go on a date,' he said.

Frisk:

Frisk almost couldn't believe what was happening as she and Sans walked to the surface. It seemed as though a lot had happened in a short period of time. It was ridiculous that Sans was holding her hand and talking about buying her clothes, (where and when he had gotten his money was a mystery.) He wasn't just going to buy her things, he was _excited._ He was human. For the first time, Frisk realized that though Oliver had feelings for her, part of his unease came from how abrupt all of the changes had been for the last week and a half.

 _Maybe that's why he didn't want to have sex,_ Frisk thought. She suddenly felt guilty for being so eager.

Sans took Frisk to a resteraunt that she hadn't even known existed. Frisk often spent her time traveling back and forth between the underground and the surface, but she didn't make a habit to explore the town. Frisk liked her house and she liked keeping her distance from humans, and even monsters. Though Frisk had no troubles socializing, she simply didn't like to. It was part of what was so attractive about Sans. He didn't seem to like to be the center of attention.

He certainly liked to eat, though. Frisk's salad looked meager compared to the burger, fries and dessert that Sans ate.

'I still don't get it,' he said at one point. 'Food is one of the greatest things, why do you eat so little?'

Frisk shrugged. 'Just not as hungry.' She didn't dare admit that she liked the way Sans ate. She liked that he wasn't rail thin. She liked everything about him.

Afterwards he took her to a boutique. The two were very uncomfortable in the expensive looking store, and after a few minutes, Sans nudged Frisk and leaned close.

'There's a thrift shop like right across the street,' he whispered. 'Wanna go there instead?'

'Yeah,' Frisk said with a grin.

In the thrift store, Frisk found giant sweaters and well fitted pants while Sans looked at old baubles on a nearby shelf. He got particularly interested in a lava lamp, and Frisk tried to pay for her own things before Sans teleported next to her and paid for her clothes. She felt a little guilty for it.

They walked outside after the sun had set to find dozens of people thronging in the streets. They were talking so loudly that when Frisk tried to say anything she was drowned out. Sans didn't seem to notice when she tried to speak, but he gave her a knowing look after a few moments and put his arm around her. He held her close and leaned into her ear.

'Hold tight,' he whispered.

Sans teleported while holding Frisk. A prickly sensation covered Frisk's body and in the blink of her eyes, they were standing near the hole separating the underground and the surface. The two went home holding hands, and spent the evening with Papyrus and Mettaton watching old movies.

When they went into the bedroom to sleep, Frisk had a hard time ignoring her own urges. Sans turned his back while she put on pajamas and when they laid down, Frisk was tempted to kiss him. No, she didn't want to simply kiss him, she wanted to do _everything._ As Frisk laid with Sans, she wondered how she could even tell him. How could she say: I want to have sex with you? She knew that she couldn't hold back her desires for long.


	12. Chapter 12: Insecurities

undertale: the human equation

chapter 12: Insecurities

Frisk:

Frisk groggily opened her eyes. Sans hadn't let go of her all through the night, and though it was sweet, she was hot because of it. She stirred and tried to move, but Sans's limbs were heavy. Frisk simply lay for a while, even when he stirred, too. Almost as soon as Sans move he grumbled and got an erection. Frisk blushed, at first, and then held back a laugh.

 _He really does get hard easily,_ she thought with a suppressed giggle.

'You awake?' he asked softly.

Frisk nodded.

'Ah,' Sans said. 'Sorry,' he added, after realizing that he wasn't exactly calm. He seemed uncertain for a moment before he squeezed her tightly and nuzzled against her. 'You feel nice.'

Frisk felt her face get hot. Sans got harder, and she found herself thinking for the first time that he was big. She didn't have anyone to compare him to, but she couldn't imagine fitting it inside of her.

'Frisk?'

'Yeah?' Frisk asked. She gripped the sheats, trying to think of anything else but the horny man holding her tightly.

'I really like waking up next to you,' he said with a pleasant little sigh.

Frisk took a deep breath. 'Sans,' she said.

'Mmm?' he asked.

'Um...how do you concentrate with a boner like that?' Frisk asked. 'I don't even have one and I can't concentrate.'

'You don't have a boner?' Sans asked.

'No, I mean,' Frisk started.

'So you _do_ have a boner,' Sans said

'What-girls can't-' Frisk started.

'Yeah they can, it's just different,' Sans said with a yawn. 'You start thinkin' dirty stuff and all the blood rushes to your-'

'I get the point,' Frisk cried breathlessly.

Sans suddenly chuckled. He suddenly moved onto all fours and straddled Frisk. He hadn't realized it, but at some point in the night Sans had wiggled out of his white shirt. He had a mischevious grin on his face and his cheeks were red. When Frisk's eyes traveled down his chest and over the curve of his belly, she could see his dick pressing against his shorts.

'Why?' Sans asked. 'Watcha thinkin' about?'

 _He just-he was acting all insecure about himself last night!_ Frisk felt as though she was panicking. Her heart hammered in her chest and she almost begged him to just have sex with her right there and then.

Sans's stared for several seconds, before his cheeks suddenly turned darker and he looked down at himself. The bulge of his erection suddenly went away as he cried: 'Where's my shirt?'

 _He didn't realize he was shirtless?_ Truthfully, Frisk hadn't even realized it until Sans had moved.

Sans suddenly moved off of Frisk and threw the blanket around himself.

Frisk didn't know what to do. She sat up slowly. 'Sans?' she asked.

'Sorry,' he started. He couldn't look Frisk's way. His stomach grumbled.

Frisk hesitated and swung her legs off of the bed. 'I...I can go make breakfast,' she said breathlessly.

'Okay,' Sans said, rubbing his chin.

Frisk hurried out of the room and closed the door. She leaned against the door for a moment. _What the hell just happened?_ Frisk thought and put a hand in her hair. _He was excited, he got on top of me, and…_

'Frisk!'

Frisk turned and to her surprise she saw Undyne climbing up the stairs.

'Undyne,' Frisk said. 'What are you doing here?'

'I was going to give Paps a training lesson.' Undyne chuckled. 'I think he's still asleep though. I thought I should head back to Waterfall and let him come running to my house after he realizes that he's missed training.'

'Undyne, that's cruel!' Frisk cried.

'He's the one sleeping,' Undyne said with a cackle. 'Besides, he's got to learn how to take care of himself. That includes waking up on time.'

'I may have been keeping Sans from him,' Frisk said scratching her head uncertainly. She felt awful about what had just happened.

Undyne tilted her head to one side. She suddenly punched Frisk's shoulder playfully. 'You know, I think _you_ need some training today, Frisk.'

'W-wait, I was going to make Sans breakfast,' Frisk started.

'He can make himself food,' Undyne said and grabbed Frisk's wrist. 'Come on,' she said happily, pulling Frisk down the stairs and out of the house.

 _I was hoping the food might diffuse whatever happened,_ Frisk thought.

Undyne took Frisk to the mushroom lit paths in waterfall. Once there, she began simply walking and talking about Alphys and herself. Frisk was dumbfounded that Undyne wanted to go out for a walk.

After a bit, Frisk found herself asking: 'How are things with you and Alphys?'

Undyne scratched her head. 'Alphys is really shy.' She suddenly grinned at Frisk. 'But it makes it really cute when I do something like kiss her.'

Frisk blushed and Undyne laughed.

'You're even reacting like her,' Undyne said. 'Careful or you'll make Sans jealous.'

 _Would he get jealous?_ Frisk was having troubles trying to figure out what exactly had gone wrong that morning. _He acted jealous about Oliver._ But had Frisk ruined everything?

'Hey, don't space out on me now,' Undyne said. 'If we're going to train properly, you need to focus.' She whipped out her phone and began texting furiously, with one hand.

'Who-who are you texting?' Frisk's heart felt like it was doing summersaults in her chest.

'I want Alphys to help us,' she said. 'She's probably asleep, but she'll get all flustered once she wakes up and head here pretty quickly.'

They got to Undyne's house and she stepped inside. Frisk waited outside with her arms crossed. She felt odd and she couldn't shake the desire to run back home and talk to Sans. He had drawn away from her; he had looked hurt, or maybe scared? The truth was, Frisk didn't know _what_ Sans had felt, only that he had drawn away quickly and sudenly.

'Alright!' Undyne said, stepping outside of her house. She carreid two tennis racks and a sack full of balls. 'It's time for some training!'

Undyne handed Frisk a racket and the two began to simply it a ball back and forth. Frisk still couldn't stop thinking about Sans and kept missing the ball. At one point Undyne put her hands on her hips.

'Alright, kid,' she said sternly. 'What's on your mind?'

Frisk blushed and shook her head. 'I'm fine,' she started.

'You've had this mopey look on your face since I found you.' Undyne rose a brow. 'C'mon,' she said with a grin. 'We've been through too much for you to hold out on me now. What's bothering you?' She suddenly got a shiny, angry look her eye. 'Was it Sans?'

Frisk felt her face get hotter and she gripped her tennis racket.

'Tell you what,' Undyne said. 'I'm gonna through the ball your way. I want you to hit it as hard as you can, and when you do, yell whatever first pops into your head.'

'Uh,' Frisk started.

'Get ready,' Undyne said and pulled her arm back. 'DON'T HOLD BACK!' she shouted and hurled the ball as hard as she could.

Frisk swung her tennis racket and shouted: 'HAVE YOU AND ALPHYS EVER HAD SEX?'

Undyne was so surprised that the ball went right past her and she dropped her racket. After a moment of awkward silence she let out a hearty laugh and said: 'What?'

Frisk bowed her head in shame.

'Well, yeah, it's been five years, and God knows I couldn't wait that long,' Undyne said. She suddenly got an angry look on her face. 'Did...did you and bone head have sex?'

'N-no,' Frisk started.

Undyne tilted her head with and let out a low chuckle. She stepped forward and sat on the ground. 'Sit with me, Frisk.' Frisk did as she said and Undyne took a drink out her waterbottle. 'Spill the beans. What's happened?'

'That's just it, I don't know,' Frisk said and hugged her knees. 'We...started sharing a room and he was very…'

'Horny?' Undyne asked and laughed when Frisk blushed. 'Well, I can imagine. Alphy and I don't live together yet because...well, back when we were first dating, I'm pretty sure I would have been the same way.' Undyne grinned. 'Sometimes when I see her I just can't keep my hands off of her.' She kicked a nearby stone. 'It doesn't help that she's so sweet and cute.'

'I was okay with it,' Frisk said. 'But he wanted to wait. So, that's what I was doing.'

'Whaaaat?' Undyne said. 'Geeze, I thought this would be the other way around. I always thought you'd be kind scared to have sex with anyone.' Undyne leaned forward curiously. 'So...what, did you try something? Did you make him uncomfortable?'

Frisk sighed. 'This morning...we were just talking, and I think we were both horny,' Fris said. She put her hand in her hair, fighting through her embarassment of her admission. 'He was joking around and he got on top of me. But...see, he had his shirt off and as soon as he realized it, he just...drew away from me.' Frisk closed her eyes. She felt oddly exposed by admitting everything, but it felt nice to get it off of her chest.

'Oooooooooh, I see,' Undyne said brightly.

Frisk looked at her friend hopefully. She couldn't figure it out at all.

'He was embarassed,' Undyne said.

Frisk felt like her mind was blank. 'Well...maybe.'

'Yeah, I was the same way,' Undyne said.

'Huh?' Frisk was confused. 'I don't understand.'

Undyne smiled, despite blushing, herself. 'Well. I'm no fan of what I look like. And I'm okay with that.' She leaned back and looked up at the ceiling of the waterfall caves. 'But the thought of Alphys seeing me naked, let alone _liking_ what she saw, was completely foreign.'

Frisk studied Undyne carefully. 'You think...Sans feels that way?'

'Well, duh, he's acting exactly how I did,' Undyne said with a laugh. 'The only difference is, Alphys wouldn't leave me alone after the first time we tried anything.' Undyine tilted her head to one side curiously. 'How do you, feel, though?'

Frisk played with the dirt and rocks at her feet. 'I feel like I should leave him alone,' Frisk admitted. 'Even though I really don't want to. I want to call him...or go back home to him.'

'Then it's decided!' Undyne cried. 'We'll have a super awesome slumber party at my house with Alphys!'

'Wha?' Frisk asked.

'Your gut reaction was to leave him alone,' Undyne said clenching her fists. 'So that's what we'll do!'

Before Frisk could protest, Undyne suddenly stood.

'Alphys!' she said and opened her arms.

Alphys appeared in front of the house wearing a polka dotted dress. 'H-hi,' she said with a smile.

Alphys threw her arms around Alphys and planted a kiss on her yellow nose.

'Hi, Frisk,' Alphys said.

'We're going to have a slumber party tonight,' Undyne said.

'Oh, th-that's good, I brought some anime,' Alphys said. 'And I wanted to talk to you, Frisk.'

'What about?' Frisk said, standing.

'W-well,' Alphys looked nervous. 'I found out who started the fire.'


	13. Chapter 13: Sunshine and moon beans

Undertale: The Human Equation

-quick trigger warning here. from here on out in the story these two will probably have sex a lot. if you don't like that I don't suggest reading further. Anyway, enjoy guys-

Chapter 13: Sunshine and moonbeans

Frisk:

Frisk's brow rose. Because of her dealings with Sans, she had completely been distracted. 'Who did it?' she couldn't help but ask.

'Let's get inside, first,' Alphys said. 'I also have something for you.'

Undyne grinned. 'If it's kisses, then I demand one hundred.'

'H-hundred?' Alphys stammered.

'You're right, that's too little. I demand _one million!_ '

The three stepped into Undyne's house, and Frisk sat at the table while Undyne and Alphys giggled. Frisk was burnign to know who had hurt her home, but she knew better then to pressure the ever anxious Alphys. After she made some rasberry tea that she had brought, the three sat at the table.

'Frisk,' Alphys said. 'I haven't gone to Asgore or Toriel yet with this.'

'Why not?' Undyne asked. 'Old fluffy buns has been real wound up since the fire. He even made a speech to the monsters in case it was one us.'

Alphys sighed. 'There were finger prints on a bunch of canisters of gasoline found near your house,' she said.

Undyne noticed Alphys's discomfert. She put a hand on Alphys's arm. 'Who was it, Alphy?'

'It matched the prints we have on file for Chara,' Alphys said.

Frisk's brow furrowed. 'But...how can that be? Chara's dead.'

'That's what I've been trying to wrap my head around for the last few days,' Alphys admitted. 'And...well, the fire seemed to really bring Tori and Asgore together again.' Alphys suddenly panicked. 'N-not that the fire was a good thing!'

'I know what you mean,' Frisk said kindly.

'Well...I didn't want to bring up the memory,' Alphys said. 'Not after all they've been through. Not yet.'

'I just...how?' Frisk said. 'That doesn't make any sense.'

'Don't worry, Frisk, I'm sure we'll find out what's going on,' Undyne said. 'Although you know...no one's seen that talking flower in a long time, either.'

'Flowey,' Frisk said. She hadn't even thought about the strange creature in years. She put a finger to her chin. _Maybe Flowey had something to do with this,_ Frisk thought.

'Hey, enough with the heavy stuff,' Undyne said. 'Do you have anything new to watch tonight?'

The three cuddled together on Undyne's couch for hours, watching anime. Undyne always got excited for the fights, while Alphys and Frisk debated on which characters would date which. After a long time had passed, Frisk's phone rang. She jumped, and realized for the first time that not only had she left the skeleton brother's house, but she had left _all day._

'Oooooooooooh, wonder who that is?' Undyne cooed with a giggle.

Frisk stuck her tongue out and Alphys giggled. Frisk stepped outside to answer her phone.

'Hello,' she said.

'Kid.'

Sans's voice sent shivers though Frisk. She gulped.

'Where are you?' he asked.

'I meant to make breakfast,' Frisk started. 'Undyne showed up and took me to her house, and…'

'Ah. I see.'

'SHE'S STAYING THE NIGHT!' Undyne suddenly yelled from the window.

'Huh?' Sans asked.

'Undyne-that is-she-' Frisk was having troubles speaking.

Undyne suddenly jumped through the open window, sumersaulted, and then grabbed the phone from Frisk. 'SHE'S STAYING THE NIGHT!' she yelled with a triumphant cackle.

'Uh,' Frisk heard Sans say.

'AND GUESS WHAT, BONE HEAD?'

'Undyne,' Sans said.

'NO BOYS ALLOWED!' Undyne hung up the phone and held it up as though it were a trophy. 'AH HAHAHAHAHAHA!' she cried. 'ALPHYS! I HAVE DEFEATED A MAN! COME AND KISS ME!'

That night, Frisk spent with Undyne and Alphys. She was happy that Sans had called her, but she was nervous about going home and seeing him again. She knew it had to happen, and knowing that Chara's fingerprints were on canisters of gasoline found near her house raised even more questions. Despite it all, the three ladies fell asleep together on the couch.

When Frisk woke, Alphys was cuddled against Undyne. Undyne seemed to have woken already and was reading a book. When Frisk stirred, she grinned.

'I saw some weird blue magic earlier this morning,' she said. 'I think someone missed you.'

'He's here?' Frisk said. She blushed. _That creepy bone head is always following me,_ she thought with a little smile. It was weird, to say the least, but she didn't mind that Sans always seemed to be watching her.

'I don't know,' Undyne admitted. 'I just saw the magic.'

Frisk touched her chin. Her heart began to thud against her chest. 'I should...I should probably head back.'

Undyne nodded as Frisk stood. 'C'mere.'

Frisk stepped forward and Undyne gave Frisk a half hug. She smelled like fish and it made Frisk giggle. 'Don't be afraid to talk to me about stuff,' she said kindly. 'It's okay to rely on me.'

Frisk nodded. 'It isn't easy,' she admitted.

'Well, Tori has a hard time talkin' about, you know, sex stuff,' Undyne said with a grin. 'I had to figure it all out alone. You don't have to feel that way, though.'

Frisk stepped outside of Undyne's home and began to trudge back to the skeleton household. As she did, she began analyzing what had happened yet again. _Sans was excited, he got on top of me…_ Frisk ran her hand through her hair as she walked. _Undyne was embarassed about her body. Maybe Sans is, too._ Frisk stopped walking when her feet hit snow. She put her face in her hands. _I was hurt._ It was the first time she had realized it since Undyne had taken her away. _I don't even know why, but when he drew away so suddenly, it hurt me._ Frisk took a deep breath before stepping forward.

When she got to Snowden, Frisk stood at the front door to the skeleton household for a long moment. She was full of uncertainty and didn't know what she would face within. What if Sans had decided that _he_ didn't want Frisk anymore? Frisk clenched her fists and reminded herself to stay determined. After all, if he didn't want to be with her, she needed to know, even if it hurt. Even if it mean five long years of abstinence had meant nothing. Frisk's lip trembled and she felt her courage falter.

'Kid.'

Frisk jumped and turned, only to find that Sans had teleported behind her. _I'm not ready,_ she thought. _I'm not ready for this!_

Sans ran a hand through his white hair. He had bags under his eyes as though he hadn't slept much. He held out his hand. 'Walk with me,' he said gently.

Frisk hesitantly put her hand in his.

Sans pulled her away from the house and the two began walking. Though they were holding hands, Frisk still couldn't help but worry that everything was crumbling and that he was about to break it all off.

'Um,' Sans said. They had reached one of the bridges crossing the miles and miles of pine treese. 'I'm sorry,' he said finally.

 _Sorry?_ Frisk's mind was reeling.

'I just,' Sans started. He seemed to be having a hard time speaking his mind. 'You know,' he tried again and then he sighed and shook his head.

Frisk stared at him, trying to find the right words to say. She thought back to Undyne and Alphys, and how comfortable they were together. _But they went through this, too,_ she thought. _They had to go through these things to come out stronger on the other side._ Frisk closed her eyes for a moment.

'Why,' Frisk started. She took a deep breath to steady herself. 'Why did you draw away from me?' she asked.

Sans drew his hand away from Frisk and put it on his stomach. 'I'm...I'm this,' he said. He refused to look Frisk's way. 'I was… scared.'

'Scared?' Frisk could hardly imagine it. She felt like she had been misreading the whole situation from the start. When Sans had asked for time, she thought that he meant that they were moving too fast, not that he was scared.

Sans sighed. 'I was scared that if you saw me naked, you might not want me anymore.'

Frisk's gaze followed the curve of Sans's stomach to his face.

'Because I'm fat, I don't look like those guys that are in the anime that Alphys watches, and I know that's what's ideal to other humans.' Sans put his hand in his hair. 'And...Frisk…'

Frisk reached out for Sans's hand. He jumped, but Frisk was determined. She brought Sans's hand to the side of her face and held it there, before closing her eyes.

'You aren't the only one who had troubles controling themselves,' she told him gently. She opened her eyes and offered a smile. 'And that didn't happen to me because I'm not attracted to you, you know?'

'I'm sorry,' Sans said. 'I panicked.'

Frisk stepped forward and put her arms around him. Sans jumped again.

'Heh,' he said. 'Getting handsy, huh?'

'I'm feeling your body,' Frisk said.

Sans's heart began beating faster, but Frisk ignored it.

'I can feel you,' she told him. 'And I don't want to stop.' After a moment, Sans slowly put his arms around Frisk. 'You...you shouldn't be afraid of me.' Frisk buried her face in his chest so he wouldn't see her. 'Besides,' she went on. 'I like your belly,' she muttered in a quiet voice.

'Hm?' Sans asked. 'I didn't catch that last part.'

'I, uh, like your belly,' she said again.

Sans froze, and stepped away from Frisk. He held her shoulders. 'What?'

Frisk blushed and turned her head. 'I like your belly,' she said, fiddling with her shirt.

Sans didn't say anything for a moment. 'Why?' he asked.

Frisk still couldn't look his way. 'Well,' she said, scratching her neck. 'It was...I sort of…' _Now I'm the embarassed one,_ Frisk thought.

'You, what?' Sans asked.

Frisk put her face in her hands. 'When...I discovered my own body, I kind of...I liked bellies.'

'Discovered your body?' Sans asked. 'Are you talkin' about masturbating?'

The word sent Frisk into a frenzy. 'Toriel said it was totally natural but she was super awkward when she would speak about it, and she just handed me a computer and said: when you feel those urges, look up whatever you like and I'll pretend like I don't know what's happening. So I did, and the stuff that people normally like wasn't really my cup of tea and then I found out that other people liked bellies and I found all this stuff on the internet and I-'

'Frisk,' Sans said. 'You mastrubate to bellies?'

Frisk could not feel any lower. She nodded into her hands, but refused to pull them away from her face.

'I mean...it's fine if you have a belly kink,' Sans said. 'I just...didn't expect that.' He offered a laugh and added: 'Normally my kinks are just in my bones. It just takes the right fix to get 'em out again, know what I mean?' He tugged at Frisk's wrists gently.

'Now I'm embarrassed,' Frisk moaned.

Sans chuckled. 'Well, the good news is that I guess I fit right into your belly kink, so we've got that going for us.' He pulled Frisk's hands away from her face. 'Atta girl.' He looked at her hands in surprise when he saw that she was shaking. 'What's wrong?' he asked.

'I...I thought I had done something,' Frisk admitted. 'I was...worried.'

'No, no, no, no,' Sans said. 'Frisk, it was all me,' he said. He leaned forward and pressed his forhead against Frisk's. 'I thought _I_ ruined things when you left.'

Frisk shook her head.

'Hey,' he said. 'It's kinda cold here. Maybe we should head back inside, eh?'

Sans teleported the two back into the house, which was strangely quiet. She looked around curiously as the tingling from teleporting subsided.

'Where's Paps?' she asked.

'He and Mettaton went out to see Tori,' Sans said scratching his head. 'I think they kinda like each other.' He grinned and trudged up the stairs.

Frisk was still uncertain, but she followed. 'Do you think they went out on a date?' she asked. The idea of Mettaton and Papyrus together was strange.

Sans shrugged and stepped into his room. Before he entered, he turned to Frisk and held out his hand.

Frisk felt like something important was happening, but she didn't know what. Sans pulled her into the bedroom and her brow rose.

He had put dozens of candles all over the place and lit them. He stood still for a moment and scratched his head. 'I thought...I thought maybe…' He turned and moved his hand and the door shut behind Frisk. 'I was thinkin'...we...tonight would be special.'

Frisk understood as soon as she saw the candles what was happening but she almost didin't believe it.

'I mean...it's my first time, you know,' he added. 'And...with everything that happened…' he ran a hand through his hair.

Frisk stepped forward and kissed Sans. It was brief, and when she pulled away, he stroked her neck gently.

Sans suddenly bent forward and kissed her harder. He put his hands in her hair and stepped forward. His leg caught Frisk's and the two suddenly fell. Frisk let out a squeak and Sans's blue magic surrounded the two and for a strange moment they were floating in mid-air. She suddenly landed on Sans's bed, and he on top of her.

Sans's soft, warm body pressed against Frisks, though once more he had gotten an erection. He shed his hoodie and tossed it aside. He began kissing Frisk over and over again, each time moving his body all over Frisk, sending urges coursing through her. She kissed him back, and stuck her tongue in his mouth. Sans drew away in surprise and laughed, before kissing her again. Suddenly, he stuck his tongue into her mouth and the two were entangled in a battle of taste buds. When both of their hearts were beating like hammers, and Frisk felt as wet as he was hard, he pulled away shyly.

'You're sure...about this?' Sans asked. 'You know...what I want to do, right?'

Frisk nodded.

Sans took Frisk's hand and brought it to his crotch. Sans began to move her hand slowly over the fabric of his shorts, so that she was stroking his penis through his clothes. 'Do you feel that?' he asked.

Frisk gulped.

'You made me that hard,' he said.

Frisk nodded again.

'You've watched porn,' he said bluntly. 'So you know...what people do in those things?'

'Sex,' Frisk said bluntly.

'Yeah. I want to do that.' Sans kissed Frisk's neck and ran his hand under her shirt. 'I want to...I want to take your clothes off.'

Frisk grabbed the edges of Sans's shirt. In one swift movement she pulled it off of him.

'Hey!' Sans suddenly sat shirtless on top of Frisk. He looked at her in surprise and touched his stomach uncertainly before chuckling. 'I guess I'm first then.' He bent forward and took Frisk's shirt in his hands.

Frisk's heart beat quickly as Sans took off her shirt. She lay beneath him, shirtless and exposed. Sans simply stared at her for a long moment, until Frisk, embarassed, asked:

'What is it?'

'Just,' Sans bent down and kissed her face. 'You're so beautiful, Frisk.'

Frisk, feelining brave, grabbed the waistband of Sans's pants and tugged on it. Sans looked down at what she was doing and laughed.

'Eager,' he said. He sat back up and took his pants off. Frisk took the time to slip out of her shorts. He didn't realize what Frisk had done until he turned back.

Frisk couldn't keep herself from looking at Sans's dick. _How the hell is that going to fit inside of me?_ she thought in a slight panick. It was large, long and thick and the tip was transluscent blue.

Sans got on top of her, his belly brushing against Frisk's. He gently put his hand on Frisk's vagina and slipped his fingers inside. Frisk gasped; no one had ever done that before.

'You okay?' Sans asked.

Frisk nodded. 'I just-wasn't expecting that.'

Sans pushed his fingers deeper into Frisk and she let out another gasp. Frisk touched Sans's shoulder as he continued to move his fingers back and forth. After a few moments, Sans removed his fingers and hugged Frisk. Then, he moved the tip of his dick to Frisk. He pressed against her and slipped his head in.

Frisk gasped; Sans was very large. His tip slipped in, but he was having trouble entering her.

Frisk was hot and sweaty. She felt splayed open like a book, but she wanted Sans inside of her. He froze, and moved his hips, but little happened at first.

'Jeeze, you're tight,' he said with a laugh. 'Just let me,' he said and pushed harder. Sans moved his hips and shoved himself into Frisk. Frisk gasped and grabbed Sans's shoulder.

Once Sans was inside of Frisk, he couldn't stop himself. He began moving his hips and his dick was so large that every movement made Frisk cry out. He moved himself so much that Frisk lost sight of everything else. She was embarassed at how loud she was being, but it seemed to fuel Sans's determination. Sans himself grunted and thrusted, moving faster. He kept getting faster, pounding into Frisk as hard as he could. It was slightly painful for her, but for some odd reason she _liked_ the pain. She had never seen the expression of concentration on Sans's face before. He was sweating and but he had never looked so strong before. It was as though Sans completely surrounded Frisk.

Frisk's back arched as a feeling of extreme relief flooded her. Sans made a little sound and pulled himself out of her. All at once, he ejaculated while holding his dick in his hand. He put his hand on the bed beside Frisk and froze, panting. When he finished, he grabbed his discarded white shirt, cleaned himself off and flopped onto the bed next to Frisk, panting hard.

Frisk lay still, her heart pounding in her chest, staring at Sans. She wanted to remember every curve of his body, forever.

Sans's hand crept over Frisk's. He held it tightly and closed his eyes. Niether he, nor Frisk really had much to say. Sans put his hand on Frisk's waist.

'I love you,' Frisk said.

'I love you, too,' Sans said.


	14. Chapter 14: Poker nights

Undertale: The Human Equation

Chapter 16:

Sans:

Frisk, Frisk, Frisk. Sans could not get enough of her, so much that when he fell asleep he dreamed of her. She was laying beneath him, naked, and begging him to put himself inside of her. In the dream, he gladly did as he asked, relishing in being inside of her. Every time he moved she made all sorts of noises that he had never heard her make before and made him harder. His desire was like a deep hunger that couldn't be sated. He wanted to know all of Frisk's buttons, he wanted to figure out what made her horny, what made her moan and twist and cry out.

When Sans woke, he was still naked and so was Frisk. She was fast asleep and though Sans was as hard as the earth was round, his heart melted at the sight of her. Then his gaze traveled down her body and noticed blood surrounding her thigh. It was only a few drops, but enough to be noticable.

Sans suddenly sat straight up, filled with hurry. His erection was gone in an instant. He gently shook Frisk's shoulder. 'Hey,' he said quickly.

Frisk groggily opened her eyes. 'Mmm?' she said.

'Uhm, Frisk, are you alright?' he asked.

Frisk looked down at her thigh and blushed. 'Oh,' she said. 'I-'

Sans moved off of the bed and Frisk did too. Frisk's legs wobbled and she stumbled forward. Sans caught her and would have laughed had he not been worried.

'Frisk, it isn't your time of the month, is it?' he asked.

'No,' Frisk said. 'It-it was my first time,' Frisk told him. 'Um. You popped my cherry.'

 _Oh._ Sans suddenly laughed. 'I forgot,' he admitted. 'I forgot that happens.'

'My legs are like jelly, though,' Frisk said with a giggle.

Sans held Frisk's face in his hands, and stroked her cheek. He couldn't describe the happy feelings rolling through him. He had dozens of questions he wanted to ask, but all of them felt like far away clouds above an endless paradise. He leaned forward and kissed Frisk. The heat of her mouth and the warmth of her body brought him more joy then he had thought possible.

'Shower,' Frisk said when Sans pulled away.

'What? Oh,' Sans said with a chuckle. 'Yeah. Shower.'

Frisk wobbled away from Sans and bent over to pick up her clothes.

Sans stared; he couldn't help it. _Nice,_ he thought when she unintentionally gave him a full view of her backside as she bent over the bed. He suddenly turned his head, blushing.

'You, uh, want company?' he asked sheepishly.

'While showering?' Frisk asked.

Sans scratched the side of his neck.

'I don't mind,' she admitted. 'I'm going to check if Paps is back,' she said.

Sans's stomach grumbled and he felt his blush deepen. _What the hell is wrong with me?_

Frisk paused at the door and looked his way. 'Shower and food,' she said through her own rosy cheeks. Then she turned away and Sans was left alone in his room.

The memories of the previous night were flooding his mind. He waived his hand a his blue magic put out the remaining burning candles. _We had sex,_ Sans thought and sat down on the bed. _She has a belly kink._ Sans rubbed his face, still trying to wake up, but he felt his face get hotter. He rubbed his gut, trying to piece together what he thought and how he felt. _Do I have any kinks?_ Before Sans had become human, he had been aware of all the porn floating around but he hadn't really been a sexual creature. He didn't have a dick back then and, like Paps, he just didn't think about sex. _What the hell does it even mean to like bellies?_ Sans heard the rush of water in the bathroom.

Sans went to his computer and turned it on. Out of curiosity he simple typed 'belly kink' into a search engine. He browsed a few articles, reading about feederism and finding himself falling down a rabbit hole of men and women getting off to people eating. It was the most bizzare thing that he had ever read, and he had no desire to turn Frisk into a chubby little feedee. Once he clicked on the images of the search engine, Sans stared at everything in awe, realizing that not only did he fit _right in_ with what turned Frisk on, but that he was not as large as what some people were attracted to. He found himself trying to imagine Frisk masturbating to what he was seeing and then shook his head. Sans closed his browser and stepped out of his room.

He had every intention of joining her in the shower, and he was fairly certain that Paps hadn't come home yet. He stepped into the steamy bathroom, empowered by his new knowledge. He knocked on the wall twice to announce his entrance.

'Sans?' Frisk said through the running water.

'I'm here,' he said, still blushing from looking at porn that apparently turned Frisk on. He slowly pushed aside the shower curtain and stepped into the area with Frisk.

 _Fuck me,_ Sans thought.

Frisk glistened with water as it flowed over her body. Her hair was drenched and plastered against her head. She looked at him with wide eyes before offering a smile. Every curve of her was absolutely perfect, from the little hip dips that separated her small stomach from her large, beautiful thighs, to the curve of her small breasts sitting on her ribs. All at once, Sans felt his loins react, but he tried to turn away to hide it.

'I'm almost done, bone head,' she said with a laugh.

Sans grabbed her hand and put it on his dick. 'I gotta bone to pick with you,' Sans said.

Frisk blushed. 'In-in the shower?' she asked.

 _Shit, does she not want to?_ Sans was horny and panicking. The hot water running over his body wasn't helping him calm down at all. 'Yeah,' he said. 'Uhm,' he added, 'if it's okay.'

Frisk began rubbing him with her hand.

'Ah,' Sans said. He tried to speak but could only make little sounds as Frisk touched him.

'You bruised me a little yesterday,' she whispered.

Sans laughed and shook his head. 'S-sorry,' he said breathlessly.

'It's just, it may hurt me,' she said.

Sans nodded. _Then why are you touching me?_

'But since you're worked up,' Frisk said, carefully lowering herself to her knees. 'There are other things I can do.'

'Frisk,' Sans started. He had an idea of what she was about to do, but he had never imagined that _she_ knew anything about sexual activity. He was surprised and happy all at once.

Frisk guided the tip of Sans's dick into her mouth. 'Aha,' Sans said and put his fist against the shower wall. Frisk sucked on him sending different sensations coursing through Sans. 'Frisk,' he said in a whimper. Frisk moved her mouth and tried to shove him into her throat. She convulsed for a moment, and Sans quickly pulled her mouth off of him. 'Frisk,' he said again, a little more sternly.

'What is it?' She was breathless, too.

'You don't have to,' Sans started, but was having trouble catching his breath.

Frisk stared for a moment and grinned. 'You don't like this?' she asked, stroking his penis.

He gasped and shook his head. 'No, that's not it,' he said. 'But I don't want to choke you or hurt you,' he started.

Frisk put her mouth over him once more and he gasped again. Frisk was relentlessly sucking on him and moving her mouth. With her other hand, she grabbed and stroked his balls. The sensations were unlike anything Sans had ever felt. He grabbed a fistful of Frisk's wet hair and moved her faster. She kept going, sometimes sputtering water, and she grabbed his ass to keep him moving steadily. She moved her tongue in circles around his penis as she sucked and Sans gasped and moaned because it was too much, more than he thought he _could_ feel with his dick.

Sans felt himself cumming without warning. He let out another whimper and Frisk pulled her mouth from him. He shot his load all over her face, and guiltily relished in the sight for a moment. His cum hit her cheeks and her breasts and he felt himself relax.

Frisk stood unsteadily, and let the water of the shower wash away the mess Sans had made. When she finished she simply smiled at him. 'I'm done in here for now.'

Sans stared at her, unable to speak. _That was…_

Frisk stepped out of the shower leaving a speechless Sans clutching his stomach and staring at the running water.

 _Well,_ he thought, _I liked that. Maybe blowjobs are my kink._ He put a hand in his wet hair and grabbed his soap. He gasped a little when he cleaned his penis and his balls. They were still tingling after Frisk's experiment in the shower.

Frisk:

Frisk stood in the kitchen. She certainly wanted to do it again with Sans, but her insides were aching from her first time. The ache felt weirdly good, but she knew that if he moved inside of her as he had the previous night, it wouldn't hurt in a good way anymore. She was making french toast, bacon, and sausage for food. She ran a hand through her drying hair. _Maybe that was too much?_ Frisk had mimicked something she had seen in a porn before. She wasn't sure if she had done good or bad.

'Human!'

Frisk turned and grinned as Papyrus stepped into the kitchen.

'If I had a nose, I would say it smells delicious in here,' he said and peered at the stove where Frisk cooked. 'What is this?'

'Food,' Frisk said with a smile. 'You want some?'

'I ate quite a bit with Mettaton,' Papyrus said. 'Did you know that he is fond of mochi?'

'What's mochi?' Frisk asked.

'Crushed rice! Who knew that crushing rice could make food?' Papyrus let out a _nye he he he._ He suddenly leaned forward and added: 'I also wanted to give you and my brother some space.'

Frisk blushed. 'What?' she asked.

'Well, Undyne told me that you two would eventually have sex, so I figured that I could help,' he said.

'PAPS!' Frisk cried, nearly dropping the skillet.

'What?' he asked.

'Just-I-you-,' Frisk stammered.

'It's perfectly natural, Frisk!' Papyrus laughed again and sat at a nearby table. 'Toriel was actually worried that you were still a virgin. That was why she was so insistent on you and Oliver being together. She thought he would be a nice first time, if you know what I mean.' Papyrus moved his eyebrows weirdly.

' _How many of you talk about my sex life?'_ Frisk demanded. Her face was incredibly hot.

'Oh, all of us during poker night,' Papyrus said. 'You should really come, through, half of them don't know if you like sexual things at all or if you and Sans are actually an item. We all talk about all sorts of things!'

Frisk put a hand in her hair.

'Hey Paps,' Sans said, teleporting into the kitchen. 'How were things with Mettaton?'

'Wonderful! He helped me learn a lot about puzzles,' Papyrus said. 'Did you and Frisk have sex yet?'

Sans coughed and said breathlessly: 'What?'

' _Food's ready!'_ Frisk cried. She dumped the french toast onto a large plate.

Sans gave her an odd look and Frisk shrugged.

'Apparently,' she said, breathless herself, 'this has been a topic of conversation.'

Sans suddenly laughed. 'Oh, okay.'

'What's it like?' Papyrus said. 'Undyne went into great detail but it's all very confusing. And I've never wanted to do anything like that to anyone.'

Sans sat down and scratched his head. He used his magic to grave four pieces of french toast before Frisk could serve anything, and tugged at her hands with his magic, too. Frisk smiled and got her own food, before sitting with the brothers.

'You don't need to worry about it,' Sans said bluntly. 'As long as you're a skeleton, you won't have those urges.'

 _Skeleton monsters don't have the urge to have sex?_ Frisk thought curiously.

'But if you really wanna know, remember when we first started using our magic?' he asked.

'Of course!' Papyrus said. He lifted his hand and using his own orange magic, made a few pieces of french toast dance around through the air.

'It would be like if my magic and your magic fused together,' Sans said, stuffing an entire piece of french toast into his mouth.

Frisk stared in awe; Sans had put an _entire piece_ of bread into his mouth.

He chewed slowly, took a swig of milk and swallowed, before winking at Frisk.

Frisk blushed and looked away.

'Ew,' Papyrus said. 'I would never do that with you.'

'Well, we're brothers, Paps, that's normal,' Sans said with a chuckle. 'But what if Mettaton had magic like us? Would you do that with him?'

For the first time in years, Frisk saw Papyrus blush. He suddenly put his hands to his face. 'Wowie, I never thought about that before!' He thought for a moment before decidedly saying: 'No. It's too soon. And Mettaton is far too innocent for something like that.'

'Frisk this is really good,' Sans said. When Frisk looked back at him, she saw that he had cleared his plate in moments. He put his chin on his hand and grinned.

 _What is he doing?_ Frisk knew she was blushing, but she tried to act normal. 'French toast is a specialty,' she said.

'I can tell,' Sans said. 'I want more.' He waived his finger and put four more slices on his plate.

 _Is he going to eat that all?_ Frisk turned her head. _That's almost a whole loaf of bread he's eating._

'Sans, you are absolutely ridiculous,' Papyrus said. 'If you keep eating this way, you're going to become even larger then you already are!'

Frisk rubbed her neck, trying not to notice the large grin that Sans had on his face.

'Yeah,' he said, 'I guess you've got a point, Paps.' He took another bite, although his version of a 'bite' was just shoving another full piece of toast into his mouth.

'Well. Anyway. Alphys announced that she's still researching the crime scene to find out who started the fire,' Papyrus said as Sans continued to eat.

Frisk, ignoring Sans stuffing his face, tilted her head to one side. 'Has she found something new?'

'No, but she hasn't revealed the fingerprint's owner. She's going to talk to Toriel today,' Papyrus said. 'And Asgore wanted to see you to help rebuild your house.'

 _My house._ 'On the surface?' Frisk asked.

'Of course! And I, the great Papyrus, will go off to help him.' Papyrus puffed out his chest. 'Of course, I'll have a room there, too. We'll make yours...soundproof.'

'We both will see fluffy buns and Tori,' Sans said, ignoring Paps's comment. He took Frisk's hand. 'You don't mind if I come, too, do you?'

Frisk shook her head, though she couldn't keep her eyes from traveling to his gut, which had begun pressing out distinctly against his white shirt. She gulped and said: 'Of course not.'

He grinned again and put another piece of french toast in his mouth with a wink.

Frisk turned away sharply. 'D-did Alphys say why she didn't share the evidence?'

'Nope,' Papyrus said. 'Oh my god, Sans, how much are you going to eat?'

Frisk suddenly stood. 'I'm going to put on something other then pajamas and get ready to go,' she said a little too loudly and quickly left the room.

Sans simply laughed. 'I'm still hungry, though.'


	15. Chapter 15: On the other side

Undertale: The Human Equation

Chapter 15: Sans's devious plan

Sans:

'Seriously, Sans, your stomach is sticking out of your shirt,' Papyrus said.

Sans looked down in surprise and laughed. 'Don't worry about it, Paps,' Sans said. He had eaten far more then he ever really had before, and he almost couldn't move because of it. Despite that, he had things to do, and he forced himself to stand. 'And, um...thanks for giving us space.'

'Well, I didn't want to hear what was going to happen,' Papyrus said. 'It sounds gross.'

Sans laughed and stepped out of the kitchen.

Frisk dressed in a large sweater and shorts. Her hair was a little dissheveled and Sans momentarily wondered if she had relieved herself, but then decided that she didn't have enough time. _Good,_ he thought. _I am going to see what makes you tick, Frisk._ Sans felt devious but he couldn't help it. He had thought he was the only one having troubles. Even though Frisk had told him that she wanted him, too, it wasn't as obvious. Sans finally had a card to play in their little love game, and he was definately going to use it.

He took Frisk's hand and they stepped into Snowden.

'Man, I'm full,' Sans said, rubbing his stomach. He had glimpsed a couple of images of men with distended stomachs, rubbing them and talking about them. He had no idea which images Frisk was prone to look at but he decided to start with rubbing his stomach.

Frisk squeezed his hand. 'You ate almost an entire loaf of bread,' she said with a laugh. She was trying to hide her nervousness.

Sans grinned. 'I probably coulda eaten a little more,' Sans said. 'I'm still hungry.'

'You just said you're full,' Frisk pointed out.

'Yeah, but I still want to eat.' Sans shrugged. 'It's fun.'

Frisk chuckled nervously as they stepped into the ruins. 'Sans,' she said.

 _Uh oh, that's the serious voice,_ he thought. He rubbed his stomach gingerly. He actually _was_ having troubles with how much he had eaten. His stomach was grumbling and churning in protest, and he felt like a swollen balloon. He practically had to waddle because of how full he was, which wasn't normal for him.

'Alphys told me what she found,' she said.

Sans nodded. That had been a bad night for him, but he wouldn't admit that to Frisk. With her gone, the house had suddenly felt too large and empty for him. He spent most of that night pacing the living room, trying to decide whether to call her a dozen times or none at all, and ultimately ended up teleporting to Undyne's house at five in the morning to peek and make sure she was okay. Undyne may have seen him, and he had retreated home. He rubbed his stomach; it had started to ache.

'She said that the finger prints she found matched Chara's,' Frisk said.

All thoughts of making Frisk horny suddenly died and Sans froze. He stared at Frisk wide-eyed for a long moment. 'But...how?' He was suddenly tryiing to through all the different timelines, to figure out if somewhere, somehow, Chara had survived.

Frisk shook her head. 'I don't know,' she admitted. 'But...well, it's been years since I've seen Flowey. I can't help but think...well, maybe something bigger is going on here.'

 _I thought it was Oliver,_ Sans thought, scratching his head. He tried to figure out just how Chara could be alive, when Frisk's gaze slowly drifted down his torso, to his distended stomach. Sans suddenly grinned and Frisk turned away, realizing what she had done. 'I'm sure we'll figure it out,' he said, stretching his ribs and sticking out his stomach. 'I mean, if Chara's alive, I'm sure he'll come for you, first.' A dull pain erupted in Sans's stomach and before he could go on tormenting Frisk by drawing attention to his stomach, he clutched his gut and bent over.

'Sans?' Frisk asked.

'I'm fine,' he said, grinning through the pain. 'I, um...I think I ate too much.' _And walking around isn't helping._

Frisk looked genuinely worried. 'Sit down,' she said. 'I'll see if I can find some water, or something.'

Sans sat down, but he used his magic to keep Frisk from leaving.

'Let's just...let's just sit together,' he said. He rubbed his stomach. 'Sorry,' he muttered. His devious plot to make Frisk horny was backfiring and he felt like an idiot.

Frisk hesitatntly watched him for a moment. She put her hand on his stomach.

Sans blusehd as she pushed her hands beneath his shirt. 'Kid, what...what are you doing?' She was caressing his swollen gut, and it felt weirdly _good._ His penis even hardened a little.

Frisk's brow was furrowed and her cheeks were red. 'You did eat too much,' she said accusingly. 'I...I heard that this was a good thing to do if someone eats too much.'

Sans didn't know whether to be embarassed or pleased. She moved her hands gently, and it soothed the pain. He let out a sigh when the pain released its self. Her hands were so small and soft, it was hard to believe that those same little things could make him hard and make him cum. He suddenly grabbed her hands.

'I'm fine now,' he said gently.

Frisk pouted. 'I know what you were trying to do,' she said.

Sans's brow rose.

'Sans, it...don't do that,' she said earnestly. 'Being big is unhealthy. And you could really hurt yourself by eating like that.'

She was so cute. She was horny as hell, Sans could tell; her pulse was quick and her cheeks were read and she was stammering and hesitating. But despite it all, she wanted what was best for Sans.

'But...you like it,' Sans said.

Frisk sighed and closed her eyes. 'You don't have to cater to my weird fetishes.'

Sans moved her hands so that they rubbed his stomach again. 'What if I want to?'

Frisk's eyes fluttered open. 'I don't...I don't want you to get hurt.'

Sans's stomach made a loud sound. He blushed and looked down. 'You know, this gut is always interrupting the best moments I have with you.'

Frisk smiled and planted a kiss on Sans's cheek. 'Um,' she said, 'can you walk?'

''Course I can,' Sans said stubbornly.

Frisk scratched her head. 'Well, I mean,' she started. 'You ate a ton of food. Is...is that a good idea?'

Sans suddenly grinned. 'If I get a stomach ache again, you can just rub it and make me feel better, right?'

Frisk looked like she might melt into a puddle on the floor. She turned away and stood. 'I need to see my parents.'

'Wait,' Sans said, struggling to stand. 'Hold on, I'm comin', I'm comin'.' He held his gut for a moment, panting, before hurrying after Frisk.

The two stepped through the empty ruins. Toriel was oddly absent, and a look of worry crossed Frisk's face. Sans couldn't shake his worry, either, despite having successfully flustered Frisk. He took her hand in his as they walked, urging her to move forward.

'Maybe fluffy buns is on the surface,' he said gently.

'My child,' he said kindly.

'Where's mom?' Frisk asked.

'Oh, eh,' Asgore chuckled and smiled. 'She's in the old home. Since it's her day off, I figured I wouldn't wake her to come work here.'

Frisk stared for several seconds before grinning. Sans was proud, and he was fairly certain she had learned to grin from him. 'She stayed the night?'

Asgore blushed and cleared his throat. 'It's not what you're thinking.'

Frisk laughed and looked at the house. 'When did this start?'

'Only a few days ago. It was Oliver's idea.' Asgore scratched his head again.

 _That little punk,_ Sans thought.

'It should be ready and furnished in a few more days,' Asgore told them.

'I'll go help with some of the work,' Frisk said and hurried forward.

Sans and Asgore looked at one another. 'I'll keep her outta trouble,' he said and began making his way after her. 'I bet Tori would love a good morning phone call,' he added.

Asgore chuckled and Sans walked away.

For the rest of the day, Sans used his magic to help put up walls and put down carpet. It felt good to work himself after eating so much. The feeling of being overly full was uncomfortable and as he moved and worked he felt like he was working off everything he ate. He considered the idea of finding some other way of getting Frisk horny, but he felt like he was hitting a dead end. At one point Sans was using his magic to hold up a post and he had to lift up a wall with several others. His shirt rode up but he couldn't do anything about it until the wall was secured to the house.

Sans quickly stepped away and tugged his shirt over his belly. _Alright. Gotta lose weight. Gotta ask Tori how to do that._

When Sans looked up he found Frisk looking his way from a distance. She was working with someone on a shed, but had turned and was blushing like an excited kid. Sans grinned and she turned her head, embarrassed. Sans chuckled to himself; he still wanted to lose weight but he loved how riled up he could make Frisk.

At the end of the day Asgore lit a bonfire and the company of monsters and humans gathered around, playing music and cooking food.

Sans found himself wandering into the roofless house. It was all but complete, but Sans almost preferred the endless sea of stars instead of a room. He knew, of course, that he'd regret that if it ever rained.

'Sans?'

Sans turned; he had wandered into the new kitchen. He grinned at Frisk. 'Hey.'

'What are you doing in here?' Frisk asked curiously.

'I'm wondering if maybe you'd prefer a glass ceiling or something,' Sans admitting looking back at the sky.

'Glass ceiling?' Frisk asked. 'What if it hails? It'll break.'

Sans scoffed. 'We could make it bullet proof.'

'Why do you want a glass roof?' Frisk asked.

'I like the stars.'

Both quieted and stared at the sky.

Sans had an incredible amount of power. He didn't use it to its full extent often, but he knew it was true. Hell, based off of what he knew, he was even more powerful then Tori or Asgore. Because of his low HP, though, he tried not to fight anyone. He always felt big, though, because of his power. When he saw the stars, it humbled him and suddenly he realized just how small not only he, but his whole world was. He and Frisk were mere pieces of sand floating through the universe. It made him feel alive.

'There are books on astronomy,' Frisk said.

'Huh?' Sans looked at her curiously and grinned. 'Do you mean that weird chinese thing with the goat fish and the crab?'

'No, that's astrology,' Frisk said with a giggle. 'Astronomy is the study of the stars and the planets.'

'Oh.' That _was_ interesting. 'I'd love to read those.'

Frisk nodded, getting excited. 'Are you okay?' Frisk suddenly asked.

'Whaddya mean?' Sans asked.

Frisk blushed. 'Well, you ate a ton and had to move around.'

'All digested, now. I'm good.' Sans patted his stomach. 'I got the guts to pull through whatever happens, you know? I'm real good at digesting the details of any situation.'

Frisk chuckled and shook her head, but she was blushing again.

 _Jeeze, does any attention I draw to my stomach make her bashful?_ 'So, what's on your mind?'

'For as long as I can remember being underground, I always wanted Toriel and Asgore to get back together,' Frisk said. 'I keep trying to figure out how Chara could exist, but I just can't.' She bit her lip. 'I'm going to live here.'

Sans looked around at the empty kitchen. 'I'll be here, too.'

Frisk looked at him, then, full of concern and worry. 'Are you sure?' she asked. 'Is that what you want?'

'Who else am I going to tease with my gut?' Sans asked with a grin.

Frisk's face turned red again and she shook her head. 'I'm being serious.'

'So am I.' Sans put his arm around Frisk. She wrapped her arm around him, running her hand across his belly. It made him want to grab her and kiss her, but he knew better. 'You know that, right? I don't plan on goin' anywhere.'

'What are we, anyway?' Frisk asked. 'We haven't really said…'

'Boyfriend and girlfriend?' Sans asked. 'Seems kinda cheesy to call us that.' He winked at Frisk. 'I don't know about you, but I feel like we're a little deeper then that.'

Frisk blushed again and rubbed his stomach. The sensation sent chills through Sans.

'Partners. Lovers. Comrades.' Sans grabbed Frisk's hand suddenly and she looked at him in surprise. 'Uhm,' he said. 'If you keep goin', we're probably going to end up doing sexy shenanigans,' he said with a chuckle.

Frisk simply smiled and laid her head on his belly. 'I'm glad you're here...partner?'

Sans chuckled. 'Yeah. You're more than a girlfriend to me, Frisk.' He stroked her hair. 'That's why...I denied you all those years ago. I knew that if I started somethin', I wouldn't want to stop, and it would mean everything to me.'

'Skeletons don't feel sexual urges,' Frisk said suddenly.

'Yeah, that's not really a thing without the organs to do the nasty,' Sans admitted.

'So, then...I mean, Asgore always told me there's some level of attraction,' Frisk said. 'How did you...fall for me?' Frisk lifted her head to look Sans in the eye. 'How did you know?'

'So inquisitive tonight,' Sans said. He took a deep breath and closed his eyes. 'When I first met you, I was real worried you might try and hurt Paps. But you didn't. You even went on a date with him.' Sans chuckled. 'But you were just playing along. You were making him happy by doing something that he wanted to do. That was when I was attracted to you. And...it wasn't physical, Frisk. It was all emotional. That's just how skeletons work.'

Frisk lowered her head back onto Sans's belly.

'I watched you,' Sans admitted.

'Creeper,' Frisk teased.

'Hey, for all I knew, you might be out to get everyone,' Sans said. 'But you didn't do that. You saved everyone. And by the end of it all, I knew. We were all standing at the surface, and I knew, but I couldn't do anything.' Sans suddenly opened his eyes. 'When did you know?'

Frisk sighed and thought for a moment. 'When you told me about how you met Toriel.'

Sans remembered the moment. If it hadn't been for Toriel, he would have killed Frisk right away. 'Really? That was the moment for you?'

Frisk nodded. 'You spared me, because you cared about someone else. You were also trying to protect and save everyone.'

Sans sighed. 'We're two peas in a pod, I guess. Movin' faster then the speed of light.'

'We can slow down if you want,' Frisk said.

'Nah,' Sans replied. He and Frisk were dozing off. 'I'm pretty sure I'm gonna marry you someday. I love you, Frisk. I always have.

Sans was falling asleep and he knew it, but he couldn't find the will to care. Frisk's heavy breathing told him that she was in a similar state of being. Just before Sans fell asleep a shadow moved across a nearby window. He was close to ignoring it, because there were dozens of people hanging around Frisk's house. Only this shadow reminded him of Tori and Asgore. Same elongated snout, horns curling off the side of the head. Toriel had small horns, of course, and Asgore's were long and heavy looking. The shadowed figure's horns were in between. Sans curiously lifted his finger and blue magic illuminated the window.

The shadowy figure wasn't looking into the house, but the white fur and bright eyes were unmistakable. _Asriel?_ Sans thought. Asriel stepped away and Sans lowered his hand. His magic faded.

Sans put an arm around Frisk and closed his eyes. _It can't be him,_ he thought. _Asriel is dead._


	16. Chapter 16: Fluffy times

undertale: The Human Equation

chapter 16

Sans:

Sans woke when he heard trucks outside. He jerked awake and Frisk lifted her head groggily. The beeping of a truck backing up was like an alarm going off. The two looked at each other in a panic before Sans teleported them just outside of the house. They landed in the grass. Frisk clumsily fell on top of Sans, who let out an: _'Oof.'_

'Sorry,' Frisk started, raising herself off of Sans.

Sans simply laughed, but the two looked to her house to see that the construction men and women were working on the roof. A shadow flickered just around a corner of Frisk's new house. Sans sat up slowly, trying to see. He felt something he hadn't felt in a long time.

It was the familiar tug of someone _saving_ and manipulating time. Sans stared at where the shadow had been for a long moment. _Chara,_ he thought, _is that you?_ None of the other monsters knew about Chara's malicious nature, except perhaps Asriel, who, as far as Sans knew, was _also_ dead. Things were making less and less sense, and it worried Sans.

Frisk stood and looked around curiously. 'What time is it?' she asked aloud.

The sun hadn't even hit the horizon. Sans stood with her. 'Too early,' he said, rubbing his eyes. 'Whaddya say about heading back to the house?' he asked.

Frisk nodded. She looked just about as tired as he felt.

Sans teleported the two to the bottom of the hole, rather then using the stairs. Frisk shivered, which was normal. She wasn't used to teleporting like Sans was, and it even made Sans queasy if he had eaten too much.

They walked through the ruins and into Snowden. Frisk seemed too sleepy to talk and Sans felt the same. They stepped into his house quietly. Papyrus and Mettaton were fast asleep on the couch, with the credits of some movie playing. Sans grinned at his brother, once again marveling at how close he and Mettaton were getting. Frisk hardly noticed as he pulled her up the stairs and into his room.

Frisk removed her shoes and collapsed onto the bed with a sigh. Sans sat down beside her, just watching as she fell back asleep. He reached forward and stroked the side of her face.

'You awake?' he whispered.

'Mmm,' Frisk responded.

Sans continued to run his hand over her face, and to her shoulder. 'I love you so much, Frisk.' Sans stilled his hand on the dip of Frisk's back. 'I wish we could stay this way forever.'

Frisk rolled onto her back and offered a smile, though she kept her eyes closed.

'If...if anything happens,' Sans said, 'I'm gonna protect you. I won't let anything happen.' He laid next to her and took her hand in his. 'Even if...even if we had to separate. I can't imagine loving anyone else.' Sans closed his eyes. 'I love you so much,' he said again, as he, too, fell asleep.

Frisk:

Frisk was running through a field of golden flowers. Everywhere she looked, she saw nothing but shadows and it frightened her. She was being chased, but she didn't know by what. She continued to run through the golden flowers when suddenly a root rose from the ground and caught her foot. Frisk let out a cry and fell forward, into the flowers. She struggled to sit up, when she hurt something strange behind her. She whirled around and saw a figure standing among burning flowers.

A tall young man with wispy brown hair stood before Frisk. His eyes were red and he wore a long, green sweater with yellow stripes. The flowers at his feet were on fire.

'Chara,' Frisk said. The name echoed throughout the cavern filled with flowers.

Chara lifted his hand, clutching a knife that glinted in the dim light. ' _Asriel,_ ' he said.

Frisk let out a cry and winced. Her shoulders began shaking uncontrollably, back and forth back and forth-

Frisk blinked, realizing that Sans was shaking her.

'Frisk,' he said. 'Frisk!'

'I-I'm awake,' Frisk stammered.

Sans let her go and she sat up and put her hand in her hair. 'You-you were wigglin' around and making weird noises,' he said. 'Not the good ones.' He chuckled and scratched his head.

'I was having a nightmare,' Frisk explained. Sans put his hands on hers, and Frisk realized she was shaking.

'Bad one?' Sans asked.

'It's alright,' Frisk said.

Sans drew Frisk forward. She nestled against his soft chest. He stroked her hair. 'Yeah,' he said, 's' alright.'

Frisk liked his softness. It masked the strength he had. Sans was an incredibly powerful man; his magic was unmatched. Yet, as Frisk laid against him, he was soft and gentle. All of a sudden, she realized something.

'You're not wearing a shirt again,' she said lightly.

Sans laughed. 'Yeah, I tend to wiggle outta that stuff at night. Don't ask me how.' He stroked Frisk's hair. 'I'm not used to all this yet,' he said. 'My body gets hot at night.' He suddenly grinned. 'How do you s'pose we fix that?' he asked, and put his hands under Frisk's shirt.

Sans's hands moved over Frisk's stomach. She suddenly laughed unintentionally and Sans froze. Frisk blushed and said: 'I'm ticklish there.'

'On your stomach?' Sans asked. He suddenly ran his hands over Frisk's stomach.

Frisk laughed and tried to bat him away. 'Sans-hahahaha-Sans-'

'For someone who likes bellies as much as you, your tum sure is sensitive,' Sans said, moving his fingers across Frisk's belly.

Frisk felt her face get hotter as she cackled and tried to bat him away. He suddenly knocked her onto her back. Frisk fell and Sans froze.

'Ah,' Sans said. He straddled her, and pushed the hair back from her face. 'I'm sorry,' he started. 'I didn't hurt you, did I?'

Frisk was still catching her breath. 'I'm fine,' she said breathlessly as Sans stroked her face. Sans's belly brushed against hers and she couldn't help but blush. 'Fine,' she said again, but having him straddle her and half naked had set Frisk's desires on fire.

'Frisk?' Sans asked. He studied her face before grinning.

Frisk blushed even more. 'W-what?' she stammered.

'Your cheeks are red,' he said, poking her face.

Frisk's hands flew to her fance. 'No,' she said.

'Then why are you hiding?' Sans asked. He lowered himself onto the bed beside her. The warmth of him, and his softness, and his strength all made Frisk blush even more. He reached for her wrists and gave them a gentle tug. 'Hey.'

'You know, I bet it's a lovely day outside,' Frisk said. 'Why don't we go for a walk?'

'No, no,' Sans said, with that same grin plastered all over his face. 'I'm good right here.' He put his hand back beneath her shirt and onto her stomach, only instead of tickling her he just rested it beneath her belly button. 'Why are you being bashful? We've already had sex.'

Frisk slowly lowered her hands.

'Atta girl,' he said. He moved his hand along the bottom of Frisk's stomach. 'So. What's on your mind this morning?'

Frisk gulped. _His hand is right above everything,_ Frisk thought. 'I was just thinking,' she said, 'that you're really handsome.'

Sans's magic flickered in his eye. 'Handsome?' He rubbed Frisk's belly. 'I think you're absolutely beautiful.'

Frisk saw a familiar hunger in his eye, one that she hadn't understood until after they had sex the first time. He wanted her.

Sand moved his hand up Frisk's shirt and caressed her breast.

'I like your belly, too,' he said and moved her shirt up. Sans kissed Frisk's stomach. 'I like every inch of you.' He moved closer, and Sans kissed Frisk's face. She could feel his erection through his trousers. Without warning he suddenly licked the side of Frisk's face.

Frisk gasped in surprise. She gave Sans a quizzical look, and to her surprise, she saw that his tongue was transluscent blue.

'You taste good,' Sans said.

 _He licked me,_ Frisk thought. Under any normal circumstances, she would have thought it odd or maybe even gross. Oddly enough, though, it seemed to have turned Sans on even more. He suddenly moved his hand over Frisk's clit. He put a little pressure on it, but stilled so that he was simply holding on to her. Frisk blushed and gripped the bedsheets.

Sans moved his fingers slowly, caressing Frisk with all the attention an artist would give a painting. She couldn't help but let out a gasp as he worked, all the while his eyes were glued on her.

'S-Sans,' Frisk gasped.

'I like licking you,' he said. 'You taste sweet.' He grinned, and Frisk wondered why she had never seen his tongue glow before. He began to press harder and rub Frisk more aggressively. 'Maybe that's my kink,' he said. 'I don't know. Watching you eyeball my stomach is sort of great.'

Frisk closed her eyes, suddenly embarassed. _He noticed?_ It was a horrible thought, and yet at the same time she was relieved that he didn't care. She loved the curve of his belly, even though he hid it under his thick blue hoodie usually.

'But, see,' Sans said, as he continued. 'It's more then that. I like that you're turned on. And I like that you're turned on by _me._ ' Sans suddenly moved his fingers into Frisk's vagina. She cried out and Sans quickly put his hand over Frisk's mouth. 'Paps and Mettaton are in the living room,' he whispered. 'We can't be too loud.'

Frisk nodded, and Sans moved his fingers out of Frisk, and wiped them on the blanket.

'S-sorry,' she said.

'Heh,' Sans said. He slipped out of his shorts, revealing his big, hard dick. The tip was the same transluscent blue as his tongue, and even glowing. 'Um,' he said, 'is...can we?'

Frisk wiggled out of her top. She nodded quickly after and Sans chuckled again. He pulled Frisk's shorts off without warning and grinned at her surprise.

'You're really wet,' he said, still grinning. 'You're making a mess.'

'Uhm,' Frisk said breathlessly. 'S-sorry,' she said again.

Sans put the tip of his dick on her vagina and paused. He still had that grin on his face, like he expected and enjoyed everything. Frisk's heart was beating like a wild bird trapped in her chest.

'I'm excited too,' Sans said. He took one of her hands and put it on his chest. She could feel his heart beating just as hard and wildly as her own. 'See?' He studied her for a moment and kissed her forehead. Then he moved his hips and shoved the tip of his penis into Frisk. She gasped, and he was having the same problem as before. Sans was almost too thick to enter Frisk.

Sans laughed and kissed Frisk again. 'Jeeze, Frisk, I know you're tiny but this is re- _dick_ -ulous.'

Frisk snorted, and almost all of the tension left her. Sans moved his hips and squeezed into Frisk.

'I see,' Sans said. He looked at Frisk and petted her head.

'What?' Frisk asked; she was still breathless and nervous and excited all at once.

'Relax,' Sans said. He lowered himself, so that he was nearly laying on her. He wouldn't put his full weight onto Frisk, but the warmth of him comletely surrounded her. 'I'm not gonna hurt ya.'

Frisk nodded; she hadn't realized that she was incredibly tense. She took a deep breath,and as she did Sans pushed himself further into her.

When Sans was completely inside of her, he suddenly gave her an odd look. 'You nervous?'

Frisk nodded.

Sans grinned. 'Me too.' He moved his hips and his magic flickered in his eye.

Sans moved his hips faster, and put his hand over Frisk's mouth when she moaned. After a few failed attemps at quieting her, he used his magic to give her a belt to bite on. She did so gratefully because because every movement he made sent all sorts of sensations coursing through her. At one point Sans sat up and grabbed her legs, thrusting into her and trying not to make noises, himself. Frisk reached for her clit and began playing with herself at the same time. Sans's brow rose, but then he grinned and moved faster and harder. Frisk bit harder and harder onto the belt. He fit into her like a glove, but he was so large that every time he pulled back to thrust into her again, he touched everything within her.

Frisk's back arched and Sans gasped. Sans pulled out, cumming as he did. His load went everywhere, and Frisk knew that she was in a similar state. She was relieved, but it had taken nearly everything out of her. Sans leaned forward, holding his dick and panting.

'You're so,' he said, 'tight, Frisk.'

He used his discarded shirt to clean himself again and sat on the edge of his bed with his hand in his hair.

'Sorry,' Frisk said.

Sans turned and grinned. 'Well, it wasn't a complaint.' He patted Frisk's stomach. 'You alright?'

Frisk nodded, and slowly sat up.

'You're so bashful,' Sans remarked, tossing his shirt away. He reached forward and stroked Frisk's face.

Frisk touched his hand. 'I'm not used to this yet,' she told him.

Sans nodded and drew his hand away. 'Just gotta practice,' he said with another grin.

'All this aside,' Frisk said, 'I have a bad feeling.'

Sans touched her hands. 'Yeah, kid. Me too.'

Frisk hugged Sans more tightly. 'I want to see Tori today.'

'Isn't she in the old home?' Sans asked lightly.

Frisk offered a shrug.

'Let's check it out,' Sans said with a smile. 'I want to see her, too.' He paused, running his hands through Frisk's hair.

Frisk suddenly felt bashful, though didn't really understand why. 'What is it?'

Sans smiled warmly. 'I feel lucky,' he said. 'I wish we didn't have to do anything today.'

Frisk rose a brow. 'If you had it your way, what would we do, then?'

Sans sighed and leaned back onto his head, putting his arms behind his bed. 'We'd probably do it a whole lot more, watch some cheesy movies, do it again, eat some food, do it again, bother paps, do it again, and go to bed.' Sans grinned.

Frisk giggled and playfully punched his shoulder. 'I'd be so sore,' she remarked.

'That's the goal,' Sans replied.

'I'm worried, bone head,' Frisk said.

Sans nodded. 'Well, we can't just sit around naked then,' he said and put a dramatic hand on his chest. 'It breaks my heart to say it.'

The two dressed and stepped out of the old creaky house. Frisk and Sans boarded the ferryman's boat and headed for Tori's old home.


	17. Chapter 17: Word Wrestle

Undertale: the human equation

chapter 17: Word wrestle

Sans:

Sans had only been to the old home of Asgore and Toriel a few times. Sans was no king and he knew it, so he didn't pretend to want to go and see the lives of the people that ruled him. The house creeped him out as well; when he had first walked through the halls, peering into mirrors and locked doors, he felt like an outsider looking in. Everything was kept exactly the way it had been when Aserial and Chara had died. Sans understood why: Asgore wanted his old life back. He had refused to change because of it. Despite that, he could never shake the creepy feeling he got from the old memories that infested the place.

They heard Toriel talking before they saw her. Frisk froze and Sans recognized the second voice.

Oliver stood in the study, pacing and tapping his chin.

'You're sure?' Toriel was saying.

'I couldn't be more certain if I tried,' Oliver replied and paused when he Saw Frisk. Sans didn't like the way his gaze traveled up and down Frisk's body, and he found himself glaring without meaning to. Sans looked away. 'Frisk,' Oliver said. 'Sans,' he added hesitantly.

'Mom,' Frisk said.

Toriel stood and smiled. 'My child,' she said with open arms.

Frisk put her arms around Toriel and the two embraced, leaving Oliver and Sans to stand awkwardly near each other. Toriel whispered something that made Frisk blush and step away.

What did she say? Sans was dying to know, but he kept his questions to himself.

'Frisk,' Oliver said, 'there's something important I wanted to ask.'

Sans couldn't help but glare again.

'Have you ever seen a kid with red eyes?' he asked.

Toriel closed her eyes for a moment and went still.

Sans turned to Oliver, suddenly scared and worried all at once. 'Chara,' he said.

'Yes,' Oliver said, his eyes brightening. 'That was his name.'

'Did you see him?' Sans asked. He could feel his magic burning within him.

'He's at your house, Frisk,' Toriel said.

Frisk and Sans froze, staring at the queen. Sans didn't really know how much Frisk understood about timelines, but Flowey had once been able to SAVE and reload timelines. Flowey had been a mixture of Chara and Asriel. The only reason that Sans had ever learned about the SAVES was because of the knowledge passed down to him through Gaster. He didn't dare talk about it; it was something that would hurt those who had known Gaster.

'Tori,' Sans said, putting a hand to his chin. 'Where is Alphys?'

'She's at Frisk's house,' Toriel said. She looked extremely uneasy, though.

'We should head their way,' Frisk said immediately. She had that look in her eye; she was worried, but she was also determined.

Sans couldn't shake his worry, either. 'Anyone else there?' he asked.

'Asgore,' Toriel said. She suddenly lifted her arm and white magic surrounded her. A shimmering oval of white appeared near the book case.

Everyone stared in shock at the thing and Toriel shook her head. 'I should have gone with him,' she said sternly. Then she offered a smile. 'It's not something we make a habit of. It can cause quite a bit of trouble, not unlike your teleporting.'

'I didn't know you could do that, Tori,' Sans admitted. 'I didn't know you knew about my teleporting.'

Toriel winked. 'A mother knows everything.' She turned. 'Come on, everyone.'

The company stepped through the portal, and Sans dizziliy landed in the grassy feilds beside Frisk's house. He barely had time to register where he was, when he heard the clink of metal against metal.

Frisk dashed forward before anyone else could even move.

'Frisk!' Oliver cried, running after her.

Shit, Sans said.

Frisk dashed around the corner of her house. Sans cursed at his thick legs, which carried him much more slowly the lean and elegant Oliver. Around the corner, his eyes widened as he saw Asgore, holding his trident in a defensive manner. Then his gaze fell on the form of a young man in a grean sweater with yellow stripes. Tori gasped, and Sans immediately saw the glint of the blade in the kid's hand. Chara moved his arm back to strike, and Sans rose his hand, feeling his magic burn.

Frisk grabbed Chara's wrist. Sans immediately used his magic to try and separate the two, and Tori ran to Asgore's side.

Chara let out a wild growl, but Frisk held her ground.

'Who are you?' Chara cried. 'Where am I?

'Frisk,' Frisk said sternly. 'And you're Chara, aren't you?'

Chara suddenly shoved Frisk away and took several steps back. He looked around wildly, clutching his knife.

'Chara?' he said with a laugh. He put a hand in his hair.

'Why are you attacking Asgore?' Frisk demanded.

Chara stared at the ground and closed his eyes. 'I...can't remember.' He let out a frustrated moan. 'I don't understand anything that's happening!'

Sans admired Frisk's bravery as she stepped forward and put a hand on Chara's shoulder. Chara winced and looked up. 'Where's Asriel?'

'Hold it,' Sans said. He lifted his finger and his blue magic surrounded Frisk. With a flick of his wrist, she floated and landed beside him on the ground with an: 'Oof!' Oliver knelt down beside her, and it made Sans's blood boil but he ignored it. 'Why did you attack Asgore?'

'I thought he was going to eat me!' Chara cried.

'It may have been my fault,' Asgore admitted. 'I was using the trident to make the garden, and when I heard someone moving, I thought it might be an animal, or whoever set fire to the garden previously.'

'How are you alive?' Sans asked. He was using his magic to try and detect any abnormalities, but he could find none.

'I don't know!' Chara cried. He sounded scared, but Sans knew better; for all he, or anyone knew, Chara was lying.

'My child,' Toriel said gently, 'do you remember anything?'

Chara closed his eyes and put his hand in his hair. 'I remember...Asriel,' he replied. He looked remarkably like Frisk as he stood, clutching his head and trying to recall anything. It made Sans shiver. 'Asriel was...we were…' Chara let out a frustrated sound and crouched on the ground, holding his head.

Frisk stepped forward.

'Frisk,' Sans and Oliver said at the same time. They exchanged an irritated glance with one antoher.

Frisk knelt down and touched Chara's shoulder. 'I'm sure that we can figure this out,' she said gently. 'But you can't just go around attacking people.'

Chara looked up, his red eyes wide and scared.

I don't like this, Sans thought.

'Where is Dr. Alphys?' Frisk asked. Chara stood beside her, clinging to her hand.

'H-here,' Alphys said, stepping from a hiding spot around the house. She looked nervously at Chara, whose expression had gone completely blank. 'If-if it's alright, I'd like to take you to the lab.'

'Why?' Chara asked, unable to hide an irritated frown.

'Oh, um, well,' Alphys started.

'You were dead before,' Frisk said blunty. 'You and Asreil both.'

Chara stared at Frisk blankly for several secons before muttering: 'Whatever.'

'I don't like this,' Oliver said under his breath.

He and Sans exchanged a glance again.

'For once we agree,' Sans told him.

Toriel opened a portal to the lab and the company stepped through. Chara looked at the portal uncertainly, and Sans made sure to walk behind him. If he tried anything on Frisk, he wouldn't hesitate to fight the little bastard. He hated the way Chara's gaze devoured Frisk, more then he hated Oliver. At least Oliver had the decency not to touch her; Chara continued to hold tightly to Frisk's hand.

They stepped into the lab and Alphys quickly hurried away to gather some sort of gear, and make a call. Sans could only assume she wanted to call Undyne, and he couldn't blame her; Undyne would also not hesitate to act if Chara hurt anyone.

Chara sat on a sleek metal table, and Sans wondered if it was the same one he had slept on before he became human. Frisk quickly stepped next to Sans, and Sans, pleased with himself, hooked eyes with Chara.

The simple act of looking at each other sent chills through Sans's bones. He could feel something within Chara that he wasn't sure he understood. It felt malicous and hateful. Sans still kept his magic at his fingertips, half afraid that Chara would bring out his knife and start swinging again.

'Um, Chara,' Alphys started. 'I'm going to need to put you to sleep and run some tests.'

'Sleep?' Chara's voice was sharp. 'I don't want to sleep.'

'It-it's just that, I want to make sure your organs are working properly,' Alphys insisted and held up a little glass of clear liquid.

'Child,' Toriel said kindly. She put a hand on Chara's. 'You may not remember...but long ago, you were one of my sons. I, too, want to ensure your health.'

Chara's brow furrowed and he looked at their hands. He sighed and took the glass. 'Alright, but if I die it's on your head, scaly.' Chara put the glass to his lips and swung his head back. It took thirty seconds for him to pass out. Sans counted.

Alphys let out a sigh of relief.

Toriel and Asgore looked at her.

'It must have been a misunderstanding,' Toriel said to Asgore.

Asgore scratched his neck. 'But how did this happen?'

'And why were Chara's prints on the gasoline tanks?' Sans asked directly.

Toriel put a hand to her chin and looked down. Asgore looked just as worried and glanced at the sleeping form of Chara.

'Talk to him,' Frisk said earnestly.

'He may not want to talk,' Sans said.

'Try it anyway,' Frisk insisted.

Sans's worry only grew and for the first time he got irritated. 'He just tried to attack Asgore.'

'He just woke up,' Frisk replied tartly.

'You don't know that,' Sans said. 'If you do, then why were his hands all over gass-filled canisters found near your burnt garden?'

'Both of you, stop it,' Toriel snapped and Sans and Frisk jumped. 'Sans, I don't know why you don't trust him, but all we have are fingerprints. I want to speak with him as well. Frisk, knowing what we know about Flowey…' Toriel sighed. 'It isn't unwise to proceed with caution.'

'Knowing what we know?' Oliver was thouroughly confused. 'What...what exactly happened?'

Alphys rubbed her snout and spoke. 'Flowey was a creature created by accident. Sort of a fusion of Asriel and Chara.'

'It tried to kill us all,' Asgore said gravely.

'It damn near did,' Sans added bitterly. He still eyed Chara uncertainly. But I always thought it had more to do with him then anything else. He didn't say his thoughts outloud. Toriel and Asgore had been through enough.

'Both of you should head home,' Asgore said finally. 'Oliver, you too.'

'But, mom,' Frisk started.

'No. We need to speak with Chara,' Toriel said decisively. 'And it's not a conversation that needs to be shared among everyone.'

Sans eyed the sleeping boy. 'Tori,' he said, 'if he tries anything…'

'He won't,' Tori said sternly. She smiled kindly. 'This world is not a game to see who can kill who first.'

Oliver was also hesitant to go, but Tori nor Asgore were willing to listen. Both opened portals to the seprate homes. Oliver went home and Sans and Frisk trudged to his old house. Sans couldn't keep his mouth shut once they stepped through the door.

'Frisk,' he started, 'Chara…'

'Deserves a chance,' she said, turning on her heel to face him.

She's pretty, even when she's mad. The thought didn't make Sans hesitate. 'He had a chance, Frisk, you don't understand what he became.'

'What are you talking about?' Frisk demanded.

'You've only been through one timeline,' Sans started. 'You can't possibly understand.'

Frisk looked offended, hurt, and then angry all over again. 'So, what if I had that mentality when I first met all of you?' she demanded.

'Frisk, hold on a minute,' Sans said.

'What if I fought you, or the other monsters? You all looked scary, like you might kill me at any moment,' Frisk said passionately. 'But I looked past it. Even when Undyne was bent on fighting me, I did my best to spare all of you.' Frisk turned away. 'Even Asriel,' she said.

The glimmer of tears in Frisk's eyes caught Sans off guard.

'Frisk, I,' Sans started.

'I need to go on a walk,' Frisk said and hurried away.

'Frisk,' Sans said.

'I'll be back later!' Frisk was out the door before Sans could say anything more. He stood in his house for several seconds before sighing and putting his hand in his hair. I could just follow her, he thought, and then chided himself. He had done that too much, and he knew it was creepy, but he was half worried that Frisk might get hurt. She could slip on ice and hurt her head, or she could drown in Waterfall.

Or Chara could wake up, kill Tori and Asgore, and then come after the rest of us.

The thought made Sans's magic burn. He wasn't happy that Frisk had argued with him but he didn't want her to get hurt. Dammit, he thought and stepped out the door. Dammit, I'm just going to be creepy and follow her.


	18. Chapter 18: Sneaky Sneaky

Undertale: The Human Equation

chapter 18:

Frisk:

Frisk trudged through Snowden, wiping at her tears angrily. Sans didn't think she understood, but he was wrong. Years ago Asriel and Chara had fused together to become Flowey. Flowey had tried to destroy everything, Flowey tried to kill Frisk. If Flowey was a mixture of Chara and Asriel, the thirst for destruction had to come from somewhere. Asriel hadn't seemed like the sort to want to mindlessly destroy.

Frisk knew Chara was dangerous.

Despite that, hadn't Undyne been dangerous? Hadn't _Sans_ been dangerous? Hell, with the amount of magic he had, he still _was_ dangerous. Frisk didn't see her monster friends as a danger because she trusted them. She understood Sans's hesitance and fear.

She felt like _he_ didn't understand _her._

Sans wasn't a fighter, not unless he was pushed hard enough, but for the most part he kept his quiet. _Not_ fighting the monsters had been just as much of an ordeal. Frisk as an ambassador, even though her job had been very easy as of late. She didn't fight, she spoke and she tried to make people feel better. Chara had attacked Asgore, which was unnerving. He had been willing to listen to Frisk, though. When Frisk talked to him, he calmed down.

Frisk stopped walking and put her hand in her hair. _I didn't want to argue with Sans, though,_ she thought. _It's not that I don't understand._

Frisk's attention was drawn away from her thoughts when she heard footsteps. She looked around herself curiously.

 _Did he follow me again?_ Frisk's heart started beating quickly. Sans's name was on the tip of her tongue when Oliver appeared on the trail through the tall brown trees. He stopped when he caught sight of Frisk.

Oliver's foot caught on a root and he tumbled into the snow.

'O-Oliver!' Frisk hurried forward and knelt down beside him. 'Are you okay?'

'Fine,' Oliver said breathlessly. 'Just testing gravity. I'm falling over the results.'

Frisk snorted, even though the pun reminded her of Sans.

'You look like you've been crying,' Oliver remarked suddenly.

Frisk blinked in surprise. 'Oh,' she said, and suddenly stood. 'I'm alright.'

Oliver stood, too, and scratched his head. 'Want to head to the surface with me?'

'Sure,' Frisk said. 'But, why did you come down here?'

'I wanted to spy on Chara,' Oliver admitted. 'I know Tori didn't want us there, but...well, I just don't want them to get hurt.' He gave a sheepishly. 'But you don't look like you want to go that way.'

Frisk narrowed her eyes. 'You won't kiss me again, will you.'

'No, no, no!' Oliver cried, blushing. 'I learned my lesson.' He turned and began walking.

Frisk stepped lightly after her friend. She felt guilty about seeing Oliver at all, because she knew that Sans didn't like him, but at the same time Oliver had always been her friend.

As they made their way up the stairwell and onto the surface, Oliver began talking about the projects happening in the town. Since Frisk's town hall meetings had become less and less needed, he had taken it upon himself to help Tori with the town garden.

'I really hope you come and see it sometime,' he said. 'Especially since...well, since your garden is gone.'

Frisk smiled. 'I'll plant another one.'

Oliver glanced at her, as they walked alongside a setting sun. 'I didn't-that is, I assumed you were like other girls,' he told her truthfully. 'I didn't really realize how much you liked nature.'

Frisk nodded and smiled. 'Yes. It's fun.'

Oliver rose a brow.

'You can always find new things to explore,' Frisk went on. 'There are trails in the woods you can follow. And there's lots of flowers and plants there you can use for tea and making food.'

Oliver chuckled and turned away. 'Yeah,' he said, 'I didn't really know you at all.'

'You knew me for three years,' Frisk remarked.

'You can know a person for years without knowing who they are,' Oliver said with a wise nod.

'You're too young to be acting like an old wise man,' Frisk said with a playful punch.

Oliver quieted as the two walked around the rubble of Frisk's old home. 'What happened?'

Frisk trudged through her burnt garden. 'What do you mean?' she asked, hoping he wouldn't ask more.

'Frisk, you were crying.'

It made Frisk want to cry all over again. She shoved her arms into her pockets, mimicking Sans. 'I just...I don't think we should judge Chara too harshly.'

'Sans didn't trust him, did he?' Oliver asked.

'Well, I'm not saying we should _trust_ Chara,' Frisk started. 'But we shouldn't just write him off as evil.'

'Do you really think Sans is doing that?' Oliver asked.

Frisk's shoulders sagged and her heart fell. 'Well, no,' she admitted.

'So...what's the problem?' Oliver asked.

Frisk fiddled with her shirt, trying to analyze what had happened and why she had teared up and left the skeleton household. 'We argued,' Frisk said finally. She put a hand in her hair, because Sans's words had hurt her. _That's why I left,_ she thought. She tried to think everything through logically, rather then act on her emotions. 'Sans isn't wrong. Chara's fingerprints were on the gasoline canisters found near my house. And he was attacking Asgore. He isn't wrong to be wary.'

'Then why were you crying?' Oliver asked.

'Because I _know_ that!' Frisk cried. 'I know that I can't necessarily trust him!' Frisk turned away, clenching her jaw. 'It was all just a misunderstanding,' she said and put her hand in her hair. _I shouldn't have run off,_ she thought. _He's probably worried._

'Well, you should clear the air, then,' Oliver suggested. He shoved his hands in his pockets, too. 'Plus, I mean, Sans seems pretty possesive even if he doesn't outwardly show it all the time. He probably didn't like the way Chara was holding your hand or eyeing you.'

'What?' Frisk asked.

'He was checking you out,' Oliver said. 'You know. Looking you up and down because you look good.' He blushed and shook his head. 'Just...I can see why Sans wouldn't like him.' He offered a small smile. 'First argument with him, huh?'

'I don't know if I'd even call it that,' Frisk said. 'I got hurt over nothing. That's not his fault.'

'What hurt you?' Frisk regretted her words immediately as Oliver got a worried look about him.

'Don't worry,' Frisk started. 'I'm alright.' She touched her chin. 'I'm a little worried about going back to him, though.'

'Why?' Oliver's gaze flashed with anger. 'Did he do something? Damn that man, I'll beat him up.'

'No,' Frisk said, holding her hands up. 'No, he didn't do anything.' Frisk looked at her feet. 'You know...sometimes when I'm scared I just run away from things. Even if it's completely irrational for me to be scared.' She grinned. 'I'm kind of a coward sometimes.'

Oliver looked baffled at her words. 'That's not true at all.'

Frisk shrugged. 'Maybe. But I should...probably get back to him.' She began to trudge back to the stairwell.

'Hey, Frisk?'

Frisk turned.

'Um,' Oliver said, 'thanks for staying my friend. I think...we should do that for Chara, too.'

Frisk grinned and offered him a thumbs up before plodding towards the stairs.

When Oliver stepped into the snowy woods, Sans was seething. He even used his magic to trip the bastard, only to become even more irritated when Frisk went to his side. _Dammit,_ he thought, _what am I doing?_ Sans scratched his head as the two talked. _I should just go home, this is weird._ When the two went on a _walk,_ Sans got worried and angry all over again and followed them.

He was moving carefully up the stairs, making sure to keep in the shadows, which was difficult. He got worried he'd be found until they reached the surface, where he could hide behind trees. When they stopped at the burned ruins of Frisk's old house, Sans said. He touched his stomach and pouted.

 _This is dumb,_ he thought. _I shouldn't be here._

They were making small talk. Nothing incriminating, although Oliver's gaze traveled all over Frisk's body and it made Sans want to make him trip again. Then Frisk began to explain what had happened; Frisk was hurt? They had a minor argument and she was hurt? Sans began racking his brian, analyzing every detail of what he had said. _She teared up after I said she didn't understand,_ he noted. _Could it have been that?_ Frisk was passionate, and at the mention of being hurt Oliver got braver.

She wanted to go home. She felt like she had gotten too emotional; Sans could tell. The mixture of regret and worry wasn't unfamiliar to him. He just wasn't normally the cause. Then a thought dawned on him: if Frisk made her way back to the hole, she'd walk right by the tree. She'd see him. He began to panick.

Frisk made her way to the hole, and Oliver teetered off to his home town. Sans edged along the tree carefully, eyeing both humans. Oliver disappeared onto the streets of his home, and Sans looked back at Frisk.

She had paused at the top of the winding stairwell, and was looking his way.

Sans quickly leaned back, sucking his gut in and holding his breath. He wasn't quite as thin as the tree, but it was just dark enough that if anything stuck out, it could be mistaken for a shadow. He didn't let his breath out until he heard Frisk's footsteps.

Sans quietly followed her, feeling awful about spying and hurting her. He hadn't meant to, but there was no way in hell he would trust Chara. He had seen the different timelines. He knew what that child was capable off.

Sans suddenly froze and stepped behind the closest tree he could find. Frisk was gone. While he had been lost in his own thoughts, he had completely lost sight of where she was. _Well. She's probably heading home._ Even so, Sans had a weird feeling in his gut. He touched his stomach and frowned. He peered around the tree curiously, looking back the way he came.

'See anything interesting?' Frisk whispered just behind Sans.

Sans nearly jumped out of his skin. 'F-Frisk!' he cried, turning around. He was blushing and he knew it, but there was no hiding that he had been creepingthrough the woods just in front of Snowden. 'W-what's up?' he asked, trying to act calm.

Frisk simply crossed her arms and rose her brow.

'I bet you're wondering: what is he doing here?' Sans asked, trying to think of some excuse himself. 'And the truth is…' _Shit, shit, shit!_ 'The truth is,' he said again. _Shit, what do I say?_ 'The truth is,' he said again, but he didn't want to lie and he didn't know how to tell the truth.

'The truth is: you were watching me.' Frisk had a clever glint in her eye, and the corners of her mouth twitched.

 _Is she enjoying herself?_ Sans sighed. 'I'm sorry,' he blurted. _Shit I'm gonna make her more upset with me._

Frisk stepped forward, and looked him over, like she was sizing up a piece of meat.

Sans put his hands on her shoulders. Frisk's eyes widened; she hadn't expected that. Sans drew her forward into an embrace. His heart was racing, and he feared that he'd ruined everything. He could feel her pulse, he could even see her soul swirling furiously within her body, but he didn't pay much attention to that.

'I had a feeling,' Frisk said softly. 'But I thought maybe I was wrong till I saw your stomach sticking from behind the tree.'

Sans froze and cursed his size for about the millionth time since he had become human. 'I didn't mean to hurt you,' he said.

'I might have overreacted,' Frisk replied gently.

'I don't even know how I did it,' Sans started.

'Don't worry about it.' Frisk pulled away from Sans. 'You don't have to agree with me. But I'm going to give Chara the benefit of the doubt.'

'Okay,' Sans said.

Frisk nodded, as though agreeing with herself about everything.

Sans still worried that he had hurt Frisk, somehow, or that he had harmed their trust or that he had ruined everything. 'Frisk,' he said. She looked fine, but what if he was wrong? He shook his head. 'I dun like arguing.' The whole thing was exausting him the more he thought about it. 'I don't even think that was an argument.'

Frisk nodded in agreement, and put a hand on the side of his face. Sans shivered and looked at her sheepishly, feeling weak. 'I'm not perfect,' she told him. 'I'm the one that's sorry.'

 _Aw,_ Sans thought. He grabbed her jaw and kissed her fully on the mouth. He didn't want her to think that she had to run to Oliver to express her feelings. He didn't want her to get hurt. He just wanted her to _feel_ how much he felt, and so he kissed her over and over again. He even shoved his tongue in her mouth and pushed her against the tree. The enticing heat of her made him almost want to just shed his close and start doing her on the ground.

Sans pulled away and stared at her rosy cheeks. He stroked her hair, but the worry still swirled in his chest. Despite it all, he grinned. He didn't know what to say to her, but he wanted to appear as though everythiing was fine.

'You're worried?' Frisk asked.

Sans lowered his head and chuckled. 'I thought I was hiding it.'

Frisk didn't answer, but her gaze was full of question.

'Yeah, I'm worried,' Sans went on. 'I-Frisk, I don't want you to think you can't trust me, even when we argue about stuff.'

Frisk shoook her head. 'It was hardly an argument,' she said.

'That's not my point,' Sans said. 'We gotta be open with each other. Brutally. Uncomfortably. You know?'

Frisk's face was blank.

'What I mean is, if you dun talk to me about something, then you most definatley should talk to me about it,' Sans explained. He felt his cheeks darken. 'And...I'm not so good at it either. So, if there's anything you don't want to say, you should just say it. No secrets.'

'No secrets,' Frisk said, putting a finger on her chin. 'Why are you always following me?' she asked.

'Heh,' Sans laughed. 'Uhm. I promised Tori I'd keep an eye on you.'

Frisk tilted her head to one side. 'Are there things you don't want to talk about with me?'

Sans gulped, worrying that he might make her upset again. 'Y-yeah,' he admitted.

Frisk nodded, and let out a breath of relief. 'I understand,' she said. She smiled sheepishly. 'This is all really new to me. I've never been in a relationship like this.'

'Me either,' Sans admitted. He tugged at her hand. 'Let's get home.'

As they walked towards the skeleton household, a change filled the air. Sans felt it deep in his bones, and a voice rang out in his head. Though he knew the familiar feeling of Gaster nearby, what he didn't expect was a surge of magical power rolling through the woods. He didn't stop walking; Frisk wouldn't feel it. Asgore and Tori might've only felt a sight tingle, but they were too far away to feel the brunt force of magic. Gaster's presence was a protection against the magic. As Frisk and Sans walked, Gaster's voice sounded out in Sans's head.

 _'Cursed and lost, born in frost, and in the midst the devil walks, towards the world once forgot, now the devil talks and talks'._

 _What in the world is happening?_ Sans thought.


End file.
